BUG: provide automagic scope updating

This avoids need to pass local_dict/globals_dict explicity to
query/frame.__getitem__/eval
TST: attribute lookup on datetime.datetime (same name) was failing due to scope issues
TST: added same_name_scoping tests to io/tests/test_pytables.py
ENH: add subscripting to eval expressions
CLN: use numbers module for type checking in is_* methods