Added conversion methods for Julian Day

Added methods to convert between Julian day and time interval since 1970.
NSDate category with Julian Day

Created NSDate category providing methods to create an NSDate from a Julian Day and get the Julian Day from an NSDate object.
Time system conversions

TAI-TT conversions
started on UTC-TAI conversion (with constant number of leap seconds).
Added time functions for TAI and UTC

Implemented leap seconds (needed for conversion between TAI and UTC). Leap seconds are downloaded if necessary from http://www.ietf.org/timezones/data/leap-seconds.list.
Added Geocentric Coordinate Time and NSDate additions

Added support for Geocentric Coordinate Time.
Added access method (inits, class methods, and getters) for the NSTimeInterval for the supported time systems.
Added access methods (inits, class methods, and getters) for the Julian Day in the different supported time systems.
Merge branch 'refs/heads/time-functions' into develop