Impelmented negative indexing to count from the end of the sequence for IndexSequence, ArraySequence and Vector. Updated tests and type definitions to reflect this change
Merge branch 'negative-indexing' of github.com:DavidTimms/immutable-js into DavidTimms-negative-indexing

Conflicts:
	dist/Immutable.min.js
Merge branch 'DavidTimms-negative-indexing'