fixes sqlalchemy detachted session problem

the settings are now accesible with settings.<name> like before but in
addition to that usable as commandline argument together with a
docstring

For testing scripts etc. They might not work.

- two recordings of the same album with similar artist

- lay out retrieval logic and database access to retrieval module
- add MediumFormat again (gid is faked though)

not ready yet

implements a scheduler to call certain actions periodic

(except mark_outdated)

This is a recording with more than 25 releases, so simple query_by_id
does not work anymore.

not tested yet

The mbid written into the metadata can differ from the fetched mbid, so
storing it enables detection of files with old metadata.

Bug fix was mainly to comment out the MediumFormat code. This does not
work properly without changing python-musicbrainzngs.

plumber is not functional yet

fetcher will be used by two threads, making it a module
prevents two caches, locking prevents simultanious execution

- add fetcher_cache_length: maximum cache size
- add fetcher_cache_age: maximum cache age

The cleanup action for maximum cache age is not triggered yet.

- /Releases/<releasegroup>
- /Releases/<releasegroup>/<release>
- /Releases/<releasegroup>/<release>/<track>

- /Artists/<artist>/<releasegroup>
- /Artists/<artist>/<releasegroup>/<release>
- /Artists/<artist>/<releasegroup>/<release>/<track>