Initial import of echonest autotag plugin

* Drop-in replacement for chroma.py, minus submit functionality
* Requires pyechonest library, and an Echonest codegen
  binary (echoprint or ENMFP)
Optionally store echonest audio summaries in items table

The new fields are:

* echonest_fingerprint
* echonest_id
* echonest_danceability
* echonest_duration
* echonest_energy
* echonest_key
* echonest_liveness
* echonest_loudness
* echonest_mode
* echonest_speechiness
* echonest_tempo
* echonest_time_signature

TODO: Add new query type, since most of these are floats
Fix double assignment typo
Remove _echonest prefix from db fields
Add new path variables for each echonest summary field
Add new RangeQuery for querying inequalities (<, >, BETWEEN)

Queries of the following forms are dispatched to this class:

* field:0...1
* anotherfield:...0.6
* yetanotherfield:0.35...