Add support for loading/combining multiple ADAM files into a single RDD.

A new method, AdamContext.loadAdamFromPaths, loads a set of ADAMRecord files separately and then returns the union -- it should handle the case where the different files have different sequence dictionaries associated with them.

A method, findFiles, recursively searches a path for sub-directories whose names match a given regex.  We (Genome Bridge) are using these methods to load multiple ADAM files, each produced from a separate BAM that came from a single sample (but were separated by chromosome or, more generically, genomic region).

Including ReferenceMapping, and implicit ADAMRecord -> ReferenceMapping conversion.
This is used to generalize the RichRDDReferenceRecords class (renamed from RichADAMRDD, a name which was, as Frank pointed out, ambiguous).