Added SiteConfigApi, refactored other REST APIs to use updated abstract

src/main/java/org/nrg/xapi/rest/event/EventHandlerApi.java

...ava/org/nrg/xapi/rest/notifications/NotificationsApi.java

src/main/java/org/nrg/xapi/rest/settings/SiteConfigApi.java

src/main/java/org/nrg/xapi/rest/theme/ThemeApi.java

src/main/java/org/nrg/xnat/initialization/RootConfig.java