Implement lambda expressions
Merge branch 'master' of https://github.com/dropbox/pyston into lambda_expr2

Conflicts:
	src/codegen/irgen/hooks.cpp
	src/codegen/irgen/irgenerator.cpp
	src/core/types.h
Lmabda: execute review comments
Merge branch 'master' of https://github.com/dropbox/pyston into lambda_expr2

Conflicts:
	src/codegen/irgen/hooks.cpp
	src/codegen/irgen/irgenerator.cpp
Merge pull request #90 from undingen/lambda_expr2

Implement lambda expressions