Added implementation of PartitionSource with tests

Notes:

* For Cascading documentation on PartitionTaps, see: http://docs.cascading.org/cascading/2.5/userguide/html/ch08s07.html
* Implementation is very similar to existing TemplateSource
* User can pass in delimiter + pathFields, TemplateSource-style, OR a custom Partition. I've written tests for both
* Unlike a TemplateTap, a PartitionTap can be read from as well as written to. I haven't implemented this
* I wanted to add a flag to automatically discard the pathFields from the output tap prior to writing, discussed here: https://groups.google.com/forum/#!topic/cascading-user/w24BMQ6ZG10. Nobody replied so I've removed this functionality for now