Adds ability to set number of channels

to MongoConnection which are read from application.conf's
mongodb.channels property.
Merge remote-tracking branch 'upstream/master' into channels-config

Conflicts:
	src/main/scala/play/modules/reactivemongo/ReactiveMongoPlugin.scala
Merge pull request #15 from KirillGrishin/channels-config

Adds ability to set number of channels to MongoConnection