Travis support

This commit adds [travis-ci](https://travis-ci.org/) support to the java driver.  The travis build (which would be located [here](https://travis-ci.org/datastax/java-driver)) will run the maven 'test' goal from the parent module using openjdk6, oraclejdk7, and oraclejdk8.   This will only run tests in the 'unit' group (though we could extend this to run 'short' group integration tests as well).

Example result can be found [here](https://travis-ci.org/tolbertam/java-driver/builds/43388661).  You'll see that the jdk8 build failed as a unit test does not pass while it does for jdk6 and 7 (has to do with ordering of errors, I'm looking into this).

When also merging into 2.1, we will need to change the branch link at line 4 on README.rst to:

 ```rst
.. image:: https://travis-ci.org/datastax/java-driver.svg?branch=2.1
```