Label Support (and some breaking changes)

NOTE: this pretty much the first time i've written Python, so all my changes are probably terrible.

I use "label group" a lot below to refer to a list of labels that we use together logically (e.g. test+resilience and test+feature).

#### Generate Label Summary and Detail Pages

* all labels in all label groups must exist on all repos for this to work
* the pages could probably use a little work but i wanted to see if the direction was a good one first.
* pages are generated in `<outdir>/labels`
* a new argument is added to `json` command. if supplied, then label json is included in written file (i haven't actually tried generation without it but i think it should just be empty pages). for example `-l 'test,resilience;test'` will include the json for two label groups: `test+resilience` and `test`.

example screenshots:

![summary](http://public.jordanwest.me/screenshots/issues-by-label-groups-index.png)
![detail](http://public.jordanwest.me/screenshots/issues-by-label-groups-page.png)

#### Extend `list-issues` to support listing by label group

* `-m` has been removed and replaced by `-g` and `-f`. `-m <milestone>` is equiv. to `-g m -f <milestone>`
* only works for one label group across all repos (all labels in label group must exist)

example:
```
(env)$ bin/issues list-issues -g label -f "test,resilience" -t "yaya" "basho/riak_core" basho/riak_kv
===> Processing issues for repo basho/riak_core
* #563  migrate btypes_eqc to riak_test and enhance
* #485  Plug background manager's EQC tests into Eunit.
* #420  Should make_fold_req and is_request_forwardable match?
* #378  Multiple In-flight Broadcasts and Redundant Paths Break Broadcast Tree
* #377  Stale Grafts during Concurrent Writes from Diff Roots
* #373  Add Tests for Cluster Metadata


===> Processing issues for repo basho/riak_kv
* #660  allow_mult gets stuck in `true` state
```

#### Add `list-labels`, a simple summary by label group

* works for multiple label groups across all repos (assuming all labels in all groups exist on all repos)

```
(env)$ bin/issues list-labels -f "test,resilience;test" -t "yaya" "basho/riak_core" basho/riak_kv
===> Processing issues for repo basho/riak_core
| Label Group     |   # Open |
|:----------------|---------:|
| test,resilience |        6 |
| test            |        6 |


===> Processing issues for repo basho/riak_kv
| Label Group     |   # Open |
|:----------------|---------:|
| test,resilience |        1 |
| test            |        1 |
```

#### Other Changes

* rename `list` -> `list-milestones` to avoid ambiguity with `list-labels`