Added methods to calculate greenwich and local sidereal time.

This commit improves the functionality of TTimeStamp class by adding 4 methods to calculate Greenwich/local mean/apparent sidereal time. Calculation was referenced from

Aoki et. al. Astron. Astrophys. 105, 359-362 (1982), http://adsabs.harvard.edu/abs/1982A%26A...105..359A

and

http://aa.usno.navy.mil/faq/docs/GAST.php

In the future, it would be helpful to add a class that pulls the UTC-UT1 offset times from the below repository for enhanced accuracy:

ftp://ftp.iers.org/products/eop/bulletinb/format_2009/