Added AKAngularDifference

This function calculates the difference between two angles.
Merge branch 'refs/heads/time-functions' into develop
Started on Coordinate functions

Created struct for points in spherical coordinates (3D).
Merge branch 'refs/heads/time-functions' into develop
Merge branch 'refs/heads/develop' into coordinate-functions
Added celestial point struct

Added celestial point struct denoting a position on the celestial sphere.
This includes an equinox and epoch (time dimension).
Added Geographical location types.

Added AKAngle for angles in radians. Changed some utility functions to reflect these changes.
Added AKMakeGeographicalPoint
Coordinate systems added

Enumerations
Added Sidereal Time functions

Started on Sidereal Time.
Merge branch 'refs/heads/coordinate-functions' into time-functions

Conflicts:
	AstroKit.xcodeproj/project.pbxproj
Implemented Sidereal Time

Implemented a function to calculate the mean sidereal time at Greenwich.
Created and implemented Nutation

Using the full IAU 1980 theory of nutation.
Added String functions for angles

Added basic string functions to create strings from angles in either HMS or DMS format.
Added Nutation

Added calculation of nutation.
Added calculation of obliquity of the ecliptic.
Correction for nutation in sidereal time, to get from mean to apparent sidereal time.