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
[corlib] Fix implementation of Tuple hash generator

The Tuples.cs code generator dies with:

Unhandled Exception:
System.NotImplementedException: The requested feature is not implemented.
  at TupleGen.computeHash (Int32 destVar, Int32 start, Int32 count) [0x00000] in <filename unknown>:0
  at TupleGen.WriteHash (Int32 destVar, Int32 start, Int32 count) [0x00000] in <filename unknown>:0
  at TupleGen.Main () [0x00000] in <filename unknown>:0

This is caused by the incorrect definition of WriteHash(), which
instead of recursively calling itself is calling computeHash().

Thanks to Marek Safar for reporting the issue in
https://github.com/fsharp/fsharp/issues/188#issuecomment-34351850
Merge pull request #895 from ranma42/fix-tuple-hash-codegen

[corlib] Fix implementation of Tuple hash generator
Merge pull request #892 from initram/master

Implemented Parse() and TryParse() for System.Numerics.BigInteger and DaysInYearBeforeMinSupportedYear for Calendars
Added Linux travis branch