Added in PEP 8 :
"""
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).
"""

Checking for unconsistent returns corresponds to implementing a check
based on ast tree. Given an AST tree, one can easily collect return
statements and check whether they return an explicit value or not.
Also, from the AST tree, one can try to check whether the end of
the function is reachable.

Warning W740 is added for explicit inconsistent returns.

Warning W741 is added for implicit inconsistent returns : when a
functions explicitly returns a value at some point but does not
at the (reachable) end of the function. If the end of a function
is not reachable but warning is triggered, one might simply add a
"return None" or an "assert False" : as one said : "Explicit is
better than implicit.".

Regarding the code :
Implementation has been made as easy to understand as possible.
The new check itself has been implemented in a new class
UnconsistentReturns which uses yet another class FlowAnalysis
which serves as an holder for various helper methods.
Also, I took this chance to change a few details so that AST-tree
checks fit more easily. This changes a few APIs. I don't know if
anyone is relying on those.

Regarding the tests :
Adding the first AST-tree based check leads to most of the
(incorrect) test code to be parsed which leads to many SyntaxError
either related to a single version of Python or to all of them. A
A new symbol has been to be able to convey the fact that an error
is expecting only for such or such version of Python.
I've fixed all issues related to SyntaxError so that they are all
passing all right. I hope I haven't changed what is actually tested.

Comment was made that checks should be ignored by
default. This is now the case and code/documentation
and tests have been updated.

Also, Travis commented that a few things went wrong
on pretty much all Python versions. I think I have
fixed most of the issues (related to the fact that :
 - some AST elements have changed
 - sys.version_info is not a namedtuple on Python 2.6).

Also, I realised that a test would fail as a SyntaxError
only on Python 2.6. For that reason, I've made the
version tag on error somewhat more generic. Now, it can
be any prefix for a version : 2 corresponds to all Python
2 version as 2.6 correponds to 2.6 and so on...

SyntaxErrors were reported in a slightly different
place making the tests fail. Removing error location
for the 3 errors involved fixes the issue.

Conflicts:
	docs/intro.rst