Implements support for WMA/ASF

 * Adds support for importing/managing .wma and .asf files
 * Adds support for all available ASF tag equivalents
 * Adds two utility methods for (un)packing embedded ASF pictures
 * Modifies scrub plugin to work around the lack of a delete method on
   ASFTags object.
Safely cast ASFBoolAttributes

 * Modify _safe_cast to account for ASFBoolAttributes
Map a few more tags to custom/nonstandard attributes

 * Adds TotalTracks, TotalDiscs, MusicBrainz/ASIN and
 * Acoustid/Fingerpint
merge WMA support (#75)
minor style cleanup for #75
changelog/docs for WMA support (#75)
add "bogus" WMA names for nonstandard data (#75)
add WMA test file (#75)
fix crash when WMA is missing cover art (#75)
fix some tags in test WMA file
fix conversion of ASFBoolAttribute to bool (#75)
warning about _unpack_asf_image