New AdamContext and RichAdamRecord decorators. New RDD methods. Schema fixes.

o Created a new AdamContext decorator for SparkContexts with methods for
  creating new ADAM RDDs from SAM/BAM or ADAM files
o New methods added to RDDs created by ADAM for aggregating pileups, sorting, and
  saving. Makes it easy to compose transformations, e.g. rdd.sort().markDups().save()
o Added new 'RichAdamRecord' decorator for ADAMRecords with useful utility methods
o Fixed a 1-off bug in mateReferencePosition. All offsets are now 0-based.
o Removed the 'end' field from an ADAMRecord and added 'mateReferenceId'