0.9.0: Replicaset support

• Implemented #9: Reconnect - according to specification, by checking
  a connection with the ping command if it has been not used for a
  defined number of seconds (defaulting to 5)
  (@thekid)
• Merged PR #10: Sessions. Adds support for passing sessions to all of
  the database and collection methods.
  (@thekid)
• Merged PR #5: Add support for mongodb+srv. This adds support for DNS
  seed lists, as well as reading from and writing to separate connections
  based on the read preference supplied.
  (@thekid)