Implemented overloaded versions of Parse and TryParse functions for BigInteger.

Implementation is heavily inspired by Int32.
Added unit tests for the new functions.
Merge branch 'BigIntegerParse'
Merge remote-tracking branch 'upstream/master'
Cleanup and better comment.
Implemented DaysInYearBeforeMinSupportedYear for all Calendars that should have it in .Net 4.5
Merge pull request #892 from initram/master

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