Support mongoDb replica sets and local mongo _id

Use URIParser to create connections so that connections to either a
single mongo db or a replica set are supported. A replica set connection
is required in order to be able to keep inserting documents in the
replica set when the master fails over.

Add an option to automatically generate a mongo _id field using the
event timestamp when inserting a document.
Merge remote-tracking branch 'logstash/master' into improve-mongodb-output

Conflicts:
	lib/logstash/outputs/mongodb.rb
Merge pull request #389 from zimride/improve-mongodb-output

Support mongoDb replica sets and local mongo _id