changelog & style tweaks for Sound Check (#97)
mediafile: get rid of FloatValueField

At the risk of increasing line noise, I've replaced the specialized
FloatValueField (for ReplayGain tag encoding) with two more generalized options
on StorageStyle (float_places and suffix). This allows us to avoid an
unnecessary encode/decode round-trip when encoding for Sound Check.
rename Sound Check conversion functions

Now that the Sound Check functions are freed from having to deal with string
encoding (and now exclusively accept/return floats for use by the rest of
beets), I'm renaming them to reflect this.
recommendation is now a "real" enumeration