Added _Cache to rollback mappings in case of error.

Using a dict() could lead to problems in case of rollbacks.
For example, an artist A -> idA could be rolled back in case of an error
retrieving tags, but would subsist in the cached mapping.

Now _Cache is commited in handler if no error occurred.
Minor name changes. Add permanent cache store.

options.cache now has name of of permanent store of responses. Useful
for rerunning code quickly upon schema changes.
Add _Cache for friends, artists, tags.