Migrated SAMSequenceDictionaryExtractor and tests from Picard to htsjdk.
Merge pull request #121 from samtools/gg_mvagain_SAMSequenceDictionaryExtractor

Migrated SAMSequenceDictionaryExtractor and tests from Picard to htsjdk.
Added capacity to create a simple interval list from a string (the name of the contig)
Added the capacity to subtract one interval list from another (currently
it would only work if they were both wrapped inside a container)

Added some more tests