Add test group annotations to riak_test

Enable tests to be tagged with a list of attributes that can be called on the riak_test command line to group or aggregate tests. This feature can be used to group tests so that they may be categorized and run in sets. 

Test are tagged with the following attribute:

```
-test_type([bucket_types, http]).
```

Then, instead of named tests at the command line, groups are specified:

```
./riak_test -c rtee -g bucket_types,http
```

This was result in 2 matches against the above test; any match will run the test:

```
Tests to run: [{http_bucket_types,
                   [{id,-1},
                    {platform,<<"local">>},
                    {version,<<"riak_ee-2.0.0beta6-11-g5a161cf-develop">>},
                    {project,<<"riak_ee">>}]},
               {bucket_types,
                   [{id,-1},
                    {platform,<<"local">>},
                    {version,<<"riak_ee-2.0.0beta6-11-g5a161cf-develop">>},
                    {project,<<"riak_ee">>}]}]
```