Improvements to HTML doc generation

* Functions (like CBLVersion, CBLGeoPointToJSON…) are now emitted.
* Moved CBLJSON.* and CBLGeometry.* into API folder so all the public
  headers live in one place, which makes it easier for Doxygen to find
  them.
* Added timestamps to footers of pages.
API: Renamed CBLVersionString() -> CBLVersion()

Fixes couchbaselabs/couchbase-lite-api#60