Add a plugin for querying date fields against date intervals or instants.

The interval syntax is similar to that of NumericQuery.

Example: beet ls 'added:T2008..2010'
Refactor the date query from being a plugin to being part of Beets core.

Fields of the type DateType will now automatically be queried by DateQuery.
Move the date query documentation into the query reference documentation.
Merge pull request #521 from silb/datequery_plugin

Add a plugin for querying date fields against date intervals or instants...