Uses strings as actor identifiers.

Patch related to discussion at  https://github.com/jnunemaker/flipper-redis/pull/3 where it was decided that internally Flipper needs to store id's as strings. This is because some adapters (redis) are typeless blob stores. This change also helps with other datastore compatibility (i.e. Mongo where id's are non-integer).
Adapter specs now use string ids instead of integers.
No longer force passing actor instance to enable/disable.

This is related to #13. First, it allows just passing in anything that
responds to id to enable/disable and friends. Second, it wraps whatever
is passed in, if possible, and uses the wrapped value. Third,
identifier is no longer supported. From now on id.to_s will be used
everywhere.