Async mongodb driver

This pull request contains a new driver for MongoDB based on the [asynchronous driver](http://github.com/allanbank/mongodb-async-driver). The bulk of the changes are adding the required logic to the startup scripts to support the second driver. I have used this branch to run a series of [benchmarks](http://www.allanbank.com/mongodb-async-driver/performance/ycsb.html).

There are also a number of community contributed updates to the old MongoDB Driver. These include:

* Updated README with specific instructions for running a MongoDB suite.
* Updates to the configuration to now be strictly based on a [MongoDB Connection String URI](http://docs.mongodb.org/manual/reference/connection-string/) which is standardized across drivers.
* Other small fixes and performance enhancements.

This pull request will supersede the following pull requests:

* [PR-112](https://github.com/brianfrankcooper/YCSB/pull/112) - Integrates the MongoDB Connection String URI for both drivers.
* [PR-115](https://github.com/brianfrankcooper/YCSB/pull/115) - The writeConcern and readPreference are expressed via the standardized  MongoDB Connection String URI.
* [PR-131](https://github.com/brianfrankcooper/YCSB/pull/131) - Read preferences are passed via the MongoDB Connection String and replica sets are automatically detected for the asynchronous driver and will be enabled with the MongoDB Inc. driver by passing multiple servers in the URI.

I know this is a large volume of changes but they are all focused on the MongoDB driver and thought it would be easier to provide them in one batch than a bunch of pull requests. If you would prefer a bunch of smaller changes I will need guidance on the desired granularity.