Adds support for SoundCheck

 * Adds 2 functions to convert between SoundCheck and ReplayGain
 * Adds relevant StorageStyle to rg_track_* for SoundCheck
 * Adds a new packing type for SoundCheck
 * Modifies StorageStyle to accept a lang argument
 * Modifies StorageStyle to allow specifying Packed out_type
 * Modifies MediaField to write COMM frames with a lang attribute
Deal with unexpected or missing SoundCheck values

* Deals with gain values of 0 in SoundCheck files by interpreting them
  as 0.0dB rather than causing an exception
* Return default values if SoundCheck tag is malformed in some way
* Other misc changes for compactness
Merge pull request #97 from daveisadork/soundcheck

Adds support for SoundCheck
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
Add `max_rec` configuration options.

Allow users to apply a maximum recommendation to partial albums and albums
with differing track lengths/numbers.
Merge pull request #98 from mrmachine/low-rec-triggers

Add `low_rec_triggers` setting.
style tweaks & doc expansion (#98)
max_rec (#98): more lenient defaults

These defaults more closely reflect the current behavior of beets, which I
think is desirable.
max_rec (#98): fix bug w/ multiple track triggers

If both "tracklength" and "tracknumber" triggers occur for a certain album,
then the *lowest* recommendation max should be used. (With the `break`s in
there, it would just be the first trigger found.)