Implemented Parse() and TryParse() for System.Numerics.BigInteger and DaysInYearBeforeMinSupportedYear for Calendars

Implemented the versions of Parse() and TryParse() that take NumberStyles and IFormatProvider arguments. The implementation is heavily inspired by the implementaion of the same functions for System.Int32.

I have also written some unit tests to cover the functions. More may be needed to fully cover the functionality of the functions. 

Added implementation of DaysInYearBeforeMinSupportedYear Property to calendars calendars that need it from .Net 4.5. I did not mean for this to be the same Pull request, I am still new to this.