Check inconsistent return statements

[r3m0t]

This implements the same as #431 #527 (requested in #399). This is about the best that can be done without importing the ast module, and therefore this PR is much smaller.

PEP8:

Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as return None, and an explicit return statement should be present at the end of the function (if reachable).

This:

Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as return None.