Echonest plugin

Drop-in replacement for the chroma autotagging plugin employing the echonest song/identify API. Additionally, it also fetches and stores audio summary descriptors:

- danceability
- duration
- energy
- key
- liveness
- loudness
- mode
- speechiness
- tempo
- time_signature

Finally, it adds a new RangeQuery class, for queries of the form:

field:0...1.0
field:...0.5
field:0.5...

Unfortunately, for the above to work, it requires changes to the database schema. From the [discussion at google groups](https://groups.google.com/d/topic/beets-users/LVX0Z4xZqnQ/discussion), I understand this is an ongoing debate. For now, I simply prefix the fields with 'echonest_'.

Feel free to pick and choose, if you find this useful.