share/IlluminaData.py: added CasavaSampleSheet class and convert_miseq_samplesheet_to_casava function

Added support for reading and manipulating sample sheet files used by the CASAVA software, plus a function for converting a MiSEQ sample sheet file to the format expected by CASAVA for bcl to fastq conversion.
illumina2cluster/update_sample_sheet.py: updated to use CasavaSampleSheet class.
Updated ChangeLog.