Added snapshot support to ReactiveMongo Journal

- Added hadoop filesystem based snapshotting.
- Modified MongoDriver to use the applications actor system rather than spinning up its own.
- Close the driver in addition to the connection to shutdown reactive based actors on the host system.
- Removed fsync writes for performance.
- Removed commented out code.
- Added writeConcern (GetLastError) support to journal properties.
- Removed redundant connection.askClose as driver.close handles this.

Fixes #123