Implement dict.__contains__

TODO: When using {} to construct the dict, contains works. However
if the dict type is used, the runtime crashes in various ways (see
tests/dict_contains.py)
Implement tuple.__add__
Implement builtin function id()

Currently returns the address if the Box*
Merge branch 'master' of https://github.com/dropbox/pyston
Merge branch 'master' of https://github.com/dropbox/pyston

Conflicts:
	src/runtime/dict.cpp
Merge remote-tracking branch 'upstream/master'
Modified dict tests

removed dict_contains.py
fixed merge miss in dict.cpp
XOR returned value from id function by constant
Merge pull request #115 from chrisrammy/master

Implement tuple.__add__, dict.__contains__ and builtin id() function