log errors, don't crash, when calling `convert`
Add reference url in docs
update list of external plugins
Fix configuration priorities (see #513)
Refactor the date query from being a plugin to being part of Beets core.

Fields of the type DateType will now automatically be queried by DateQuery.
add amazon plugin to external plugin list
In beet modify: don't ask for confirmation if there is nothing to do
Add empty test audio files
Rewrite mediafile tests
Extract assertTags method
Simplify MediaField descriptor
Refactor get/set into StorageStyle
Add MP3 StorageStyle class
Add serialize method for StorageStyle
Add unpack method for storage style
Refactor MP4 tag decode
Refactor StorageStyle packing
Tests include all file formats
Add UFID storage style
Test empty mediafiles