Initial experimentation with Travis CI configuration

This is not intended to replace Jenkins immediately, and Jenkins will remain the CI of reference for merging pull requests in the near term.  Long term, it is possible that Travis will give us better integration with github, so we are investigating its use.

Author: Michael Armbrust <michael@databricks.com>

Closes #230 from marmbrus/travis and squashes the following commits:

93f9a32 [Michael Armbrust] Add Apache license to .travis.yml
d7c0e78 [Michael Armbrust] Initial experimentation with Travis CI configuration
SPARK-1321 Use Guava's top k implementation rather than our BoundedPriorityQueue based implementation

Also updated the documentation for top and takeOrdered.

On my simple test of sorting 100 million (Int, Int) tuples using Spark, Guava's top k implementation (in Ordering) is much faster than the BoundedPriorityQueue implementation for roughly sorted input (10 - 20X faster), and still faster for purely random input (2 - 5X).

Author: Reynold Xin <rxin@apache.org>

Closes #229 from rxin/takeOrdered and squashes the following commits:

0d11844 [Reynold Xin] Use Guava's top k implementation rather than our BoundedPriorityQueue based implementation. Also updated the documentation for top and takeOrdered.