Add lastimport plugin by @rafi

From rafi/beets-lastimport at 72c199d599ee76f2a5fa366a032ad33491f2c82c.
Safer GET parameter formulation for AS URL
Style fixes
Use MatchQuery for mb_trackid lookup
Use shared Last.fm API key
Working on issue #1028, resize images only once per album.
Updating to master to form a new PR.
I'll get used to this git thing one day. Cleaning up.
Merge remote-tracking branch 'upstream/master'
Add authentication to discogs plugin
harmonize all plugins docs

Add a configuration section that lists all options for each plugin.
List options in alphabetic order.
Mention the default value for each option.
Use same sentences to describe concepts common to different plugins eg 'auto
option, install description
lastimport (#1009) docs: translate from README
Tidy lastimport docs (#1009)
lastimport: Declare play_count type

Now you can get the fanciness of the numeric field without configuring the
`types` plugin.
Merge pull request #1040 from multikatt/discogs_auth

Add authentication to discogs plugin
discogs #1040: Standard API key, handle failure
Save Discogs token (#1040)
Docs for #1040 (fix #1027)
Fun with Sphinx extlinks
added some more storage styles for comment fields