PERF/ENH: Dramatically speed up getting "table of contents" for storage

Added keys(), __getitem__() methods to storage classes.
Simplified writing of new storage classes by providing several default methods.
API: Changed storage "keys()" to "frames" property
Merge pull request #98 from nkeim/speed_up_storage

PERF/ENH: Dramatically speed up getting "table of contents" for storage