Send a plugin event when a file is copied.

I use this event in a plugin that needs to know which file outside the
library an item was copied from during import.
Merge pull request #493 from silb/item_copied_event

Send a plugin event when a file is copied.
changelog/thanks for #493
sort items by disc and track

Now album.items() returns results in a reasonable order.
add tests for dbcore

Based on the old migration tests for Library.
give test_db a better name: test_library
dbcore: terminology

Replace "library" and "lib" references in dbcore with "database" and "db" for
short.
a bit more dbcore testing
fix migration field detection
Running `./setup.py test` works again
Merge pull request #498 from geigerzaehler/master

Running `./setup.py test` works again
Test plugin commands from plugin paths
Make plugin path configuration useable
Use default beets logger
Merge pull request #500 from geigerzaehler/logger_fix

Use default beets logger
fix infinite-recursion regression in rewrite

Reported by axion on IRC.
header on dbcore.py
reorganize dbcore into a package
tests: silence logging output
new dbcore.query module

Splitting off the Query hierarchy for easier navigation.