DDLSHELL-59. Allow comments in DDL shell

 - support shell and C-style line comments
 - support C-style multi-line delimited comments
 - Fix single-quoted regex.
 - Disable one unit-test (parser doesn't handle multi-line single-quoted strings).

issue: https://jira.kiji.org/browse/DDLSHELL-59
review: https://review.kiji.org/r/1465
Plug Avro DSL