Plug a leak in numba.types' type cache.
Fix an unrelated issue found when running test_interproc in reference leak mode
Draft for another (more complex) strategy
Merge branch 'master' of github.com:numba/numba into typeleak

Conflicts:
	numba/targets/cpu.py
Revert buggy Type modifications for now
Add Benjamin Peterson's six module
Fix type leak by using a registry of weak references
Add a commit for a potential problem
Remove copy/paste job from six (with renamed APIs!)
Fix potential pollution of _typeconv's hash table
Improve a comment and remove an unused attribute
Merge branch 'master' of github.com:numba/numba into typeleak

Conflicts:
	numba/tests/support.py
	numba/types.py
Merge branch 'master' of github.com:numba/numba into typeleak

Conflicts:
	numba/dispatcher.py
Merge pull request #674 from pitrou/typeleak

Plug a leak in numba.types' type cache.