Constant time recovery of highest sequence number

- addresses recovery performance improvements in #67
Publish replay failures to actor system's event stream

 - ref #67
Extend HBase journal documentation

 - closes #67
Fix broken link
Updated to Casbah driver 2.5.1 - support for MongoDB 2.4.
Updated readme file.
Modified journal construction. Journal now takes mongoClient: MongClient, dbName: String, collName: String.