Add ReadGroup (RG) information to the ADAMRecord

This commit adds the RG information to the ADAMRecord.

In a test with a 1GB BAM file, I found that the read group information only took about 5MB of space, e.g.
$ du -sh low_coverage.adamWith*
846M    low_coverage.adamWithReadGroup
841M    low_coverage.adamWithoutReadGroup

Most of the RG fields were null in my test BAM and required only 47 bytes of space for ~185,000 reads, e.g.

Oct 4, 2013 8:15:14 PM INFO: parquet.hadoop.ColumnChunkPageWriteStore: written
47B for [recordGroupKeySequence] BINARY: 184,929 values, 8B raw, 28B comp, 1
pages, encodings: [BIT_PACKED, PLAIN, RLE]

The ReadGroup Library and PredictedMedianInsertSize were not null and took, 23K and 75K, respectively for ~185,000 reads.

Oct 4, 2013 8:15:14 PM INFO: parquet.hadoop.ColumnChunkPageWriteStore: written
23,403B for [recordGroupLibrary] BINARY: 184,929 values, 25,218B raw, 23,380B
comp, 1 pages, encodings: [PLAIN_DICTIONARY, BIT_PACKED, RLE], dic { 2
entries, 31B raw, 2B comp}

Oct 4, 2013 8:15:14 PM INFO: parquet.hadoop.ColumnChunkPageWriteStore: written
75,484B for [recordGroupPredictedMedianInsertSize] INT32: 184,929 values,
739,724B raw, 75,461B comp, 1 pages, encodings: [BIT_PACKED, PLAIN, RLE]

The readGroupPredictedMedianInsertSize is an int and took only one bytes per 2 records on average. The readGroupLibrary is a string, "3815246", that took one byte per 8 records.