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
Whoops, got path depth wrong. Always 1
Moved into correct folder path
Fix for:
[error] /home/travis/build/twitter/scalding/scalding-core/src/test/scala/com/twitter/scalding/PartitionSourceTest.scala:43: not found: value ???
14/05/01 23:31:51 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
[error] def toTuple(partition: String, tupleEntry: TupleEntry): Unit = ???
Added support for user to specify which fields they want to include in their TSV output
Merge pull request #863 from snowplow/feature/partition-source

Added implementation of PartitionSource with tests