Marked reserved GObject class slots as private

This stops GTK-Doc from complaining
Updated .gitignore, as swe-glib.pc.in is moved to data/
Removed the changed method from GsweMomentClass and GsweTimestampClass
Added @err to function documentation
Finished documenting GsweMomentError and GsweTimestampError
Added gswe_moment_get_timestamp() to the API.
Typo fix for GSWE_TYPE_PLANET_DATA
Fixed up boxed type registration functions

* Removed their *_free functions, as they were only calling g_free
* Made their *_copy functions static
Made GsweCoordinates a boxed type
Finished documenting GsweMomentPrivate
Documented gswe_timestamp_new()
Added gswe_moment_{get,set}_coordinates() to the API
gswe_moment_set_timestamp() now increases revision, as it should be
Added more GsweMoment documentation
GSWE_HOUSE_SYSTEM_NONE is now handled at a few points
Documented gswe_moment_get_house()
Made gswe_moment_init() static
Added gswe_moment_{set,get}_house_system() to the API