Move implicit path query identification out (#649)

This is not the most elegant solution, but it does let us move most of the
query-parsing logic out of library.py. It would still be nice to get rid of
most of beets.library.get_query, but I don't yet know how to do that.
Move query parsing to new dbcore.queryparse

Fix #649.
Merge branch 'master' of github.com:sampsyo/beets