WIP: conditions working now

filtering still only ok good parsing of attributes,
subscripting, e.g df.index[0:4] works!
TST: more test changes
BUG: added HDFStore to inherit from Stringmixin
BUG: process visit_Index
ENH: use non_implemented function call in ExprVisitor

ENH: support Load context in Attribute only
BUG: fixed scoping issues by _ensure_term at the top-level
TST: fixed remaining tests

BUG: fixed Attribute ast node in a Call expression

BUG: condition with > max_selectors wasn't being handled (in filter)
BUG: py3 fixes; revise scoping rules to be more broad

     record variable states from the furthermost part of the stack
     to the most recent, overwriting if shadow variables occur
COMPAT: allow prior 0.12 query syntax for terms

    e.g. Term('index','>',5) (and show deprecation warning)
DOC/CLN: clean up a comment
CLN: refactor visitor methods into a base class
CLN: minor things
RVT: revert back experiments
TST: skip when numexpr is not installed
ENH: add resolver field to Scope objects

The interface is that the resolving object must implement a `get` method that
takes a dict key and an optional value to be used in case the key cannot be
found.
CLN: remove Value in favor of Constant
ENH: add more general dynamic name resolution
ENH: cache the resolver
TST: test complex and/or/invert criteria
ENH: convert keys to a list since we need to be able to concat them