Enable parallel test execution in console runner

Adds a `-j`/`--workers-count` parameter to `tools/packaging/test262.py`, defaulting to `[number of cores] - 1`.

Speeds up running the test suite by about ~3x on my 4-core machine, with the SpiderMonkey shell. This could certainly be optimized more by just appending test results to per-thread lists and merging them at the end, but it's better than nothing.

Add tests for zero conversion on Array#indexOf and Array#lastIndexOf

Fixes #435

Move tests for object literals

Add tests for tail-call optimization

Update contribution guidelines

It was recently decided to prefer the new `id` tag over the existing
`es5id` and `es6id` tag when authoring tests. Update the contribution
guidelines to reference the new tag.

Test262 defines tests for expression-producing syntactic forms within
the `language/expressions/` directory. Most tests for object literals
conform to this structure, but 12 such tests were added to the
`language/object-literal/` directory. Move these tests to the canonical
location for object literals.

ECMAScript 2015 introduced tail call optimization for function calls
occuring in a number of positions in the grammar. Assert expected
behavior by triggering a large (but configurable) number of recursive
function calls in these positions. Compliant runtimes will execute such
programs without error; non-compliant runtimes are expected to fail
these tests by throwing an error or crashing when system resources are
exhausted.

Fixes #477

Fix case sentitive error on test/built-ins/Object folder

Fixes gh-482

Tests for ES2015/2016 Completion Reform

Stop test262 from hanging Spidermonkey with an unimportant error message

The ES2016 draft further refines the completion values for `if` and
`with` statements. Two tests must be removed outright because the
completion value in those cases is no longer accessible from the
runtime.

In order to facilitate proper tail calls, ES2015 modified the completion
value of a number of statements.

These tests use `eval` to verify the new values.

Basic coverage for the %TypedArray% object

Replace a ES2015 test where calling the TypedArray constructor with
a floating number triggered a RangeError. Within the ES2016 specs,
the same call will trigger a TypeError, as the result for
`SameValue(NewTarget, here)` will be checked before.

Add tests for iterator expression in ForIn/Of head

Remove files that tested both PerformPromiseThen and
PromiseResolveFunction in favor of new tests that test
PromiseResolveFunction more directly and completely.

Update README.md

Although the `for..in` statement allows Expressions to define the
iterator, only an AssignmentExpression may occupy this position in the
`for..of` statement.

Improve Promise coverage: PerformPromiseAll & PeformPromiseRace

Coverage for length and name function properties

Note: Already uses the updated DataView function lengths from tc39/ecma262#266 (ES2016 Draft 2015-12-20)

Subclassing built-in objects

Improve Promise coverage: NewPromiseCapability

Improve Promise coverage: Promise Reject Function

Improve Promise coverage: Promise.prototype.then