ENH: initial commit for adding Expr based terms for pytables support
WIP: still some debugging statements in
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)