environment: using path.join, not root+'\\'+file

fixes issue #2514: jake runtests broken on mac
affects node enviroment only
Change error for index expression
Change errors for return expressions and arithmetic operands
Change assignability error to match new compiler
Improve best common type errors
Change errors related to parameters
Change errors for declare keyword
Duplicate function implementation error
Merge branch 'develop' of https://git01.codeplex.com/typescript into jsx

Conflicts:
	src/compiler/core/environment.ts