Added the "tags" command, and parsing for BAM attributes.

Added code (in Attribute and AttributeUtils) to parse attributes -- the optional fields at the end of a SAM record -- from the string which is stored in the ADAMRecord.

Updated the adam-cli project with a command, 'tags', that reads an ADAMRecord set and prints the total set of tags found in the set, along with the number of times each tag appears.
Refactored adam.avdl to have nested attributes.
Refactoring Tag to use a lazy val for attribute value conversion.
wip editing TagUtils.
wip TagUtils.