DOC: clarify docstring of rolling/expanding moments

- document center argument
- add note about the result set at right edge by default
- clarified freq keyword a little bit
DOC: fix doc build warnings
Merge pull request #6362 from jorisvandenbossche/doc-rolling

DOC: clarify docstring of rolling/expanding moments
ENH: backport Python 3.3 ChainMap

ChainMap implements a list of mappings that effectively functions as a single
dictionary. This class is very useful for implementing scope. This commit also
adds a DeepChainMap subclass of ChainMap for writing and deleting keys.
CLN/ENH: refactor Scope

The Scope class now uses DeepChainMap to look up names, and uses inspect.stack
instead of iterating over a hard coded number of frames and getting the
previous frame through the f_back attribute.
CLN/ENH: use Scope.swapkey() to update names

Also useful for replacing local variable names with their mangled versions.
CLN: update expr.py for scope changes and remove unused isexpr
ENH/CLN: track scope instead of hard coding minimal stack level
BUG: Hold temporaries created by parsing in a separate dict
TST/CLN: Add tests for new local variable API
ENH: update pytables for new scoping
ERR/API: disallow local references in top-level calls to eval
BUG: ChainMap m parameter only exists in Python 3.4

So backport that as well.
BUG: use a regular for loop in _ensure_term

In Python 3.x list comprehension build up stack frames, thus providing the
stack level to search is not the same as it is in Python 2.x.