Fix case of reporting errors with errors disabled

In some cases during type inference we could report errors, even though
type errors were disabled.

Fixes #405.
Simplify definition of abs and SupportsAbs

Also update/enable test cases.
Remove invalid test case

Closes #39.
Fix type checking of abs in Python 2

Closes #411.
Fixes to style of Python 3 builtins
Rename T to _T in builtins since it's internal
Rename S to _S in builtins since it's internal
Rename additional internal definitions in builtins
Rename internal definitions in builtins
Fixes to style of Python 2 builtins
Rename internal definitions in 2.7 builtins
Rename internal definitions in 2.7 builtins
Rename internal definition in 2.7 builtins
Rename internal definition in 2.7 builtins
Fix unittest stub
Style fixes to typing stubs
Style fixes to 2.7 typing stubs