Julian Year and Besselian Year added

Julian and Besselian Year methods added to the time functions. Included are NSDate additions for accessing standard epochs.

BUGFIX: The leap second file was opened en parsed every time leap seconds were needed. This is fixed.
BUGFIX: Leap seconds were downloaded from the internet every time the leap second file was accessed because the date of last check was not updated correctly.
Merge branch 'refs/heads/time-functions' into develop