A large update to bring into line for Dart 2, as well take advantage of newer langauge features like extension methods over top-level methods. As a result, the new API is not compatible with previous versions, but migration should be trivial.

• Added arithmeticShiftRight

• Moved into a standalone repository (outside of gba.dart).
• Added signExtend as a method to Integral.
• Added areSet.
• Added msb.

• Added signExtend

• Fixed a bug where int128 and uint128 only had a length of 64.

• Updated the documentation and README.

• Added isZero.

• Added isNegative, hasCarryBit, doesAddOverflow, doesSubOverflow, mask.
• Added parseBits.

• Add top-level isSet and isClear, Integral#isSet, Integral#isClear.
• Add checked-mode range checks to bitChunk and bitRange.
• Fix a bug in the implementation of bitChunk and bitRange.
• Added a top-level fromBits and Integral#fromBits

• Initial commit, feedback welcome!