Allow to change the default write concern for mongodb.

Add `playjongo.defaultWriteConcern` to application.conf with values
from the java drivers WriteConcern enumeration.
Merge branch 'default-write-concern' of git://github.com/magro/play-jongo into magro-default-write-concern

Conflicts:
	README.md
	app/uk/co/panaxiom/playjongo/PlayJongo.java