Support for Scalding

This patch provides scalding integration for cascading.hive.

Specifically it creates two sub-classes of FixedPathSource one for each of RCFile and ORCFile.

These classes are quire simple to use, just requiring the caller to provide field names and types as required by the underlying Cascading scheme objects. For convenience the types will default to "string" if underspecified -- which is a fairly safe default.