Add support for test group annotations to riak_tests.

- Tests can be annotated with one or more '-test_type(name)' attributes.
- Tests can be declared to be in multiple groups
- These types can be run in groups via the riak_test command line
  --groups (-g) option
- Multiple groups can be specified by comma delimiting.
Add support for multiple test_type attributes by changing format to take lists.

- For example, in a riak_test: -test_type([bucket_types, http]).
Address review feedback.

- Options --groups and --tests are mutually exclusive for now.
- fix indentation for overly long lines.
Merge pull request #674 from basho/feature/jra/test-groups

Add test group annotations to riak_test