Submodule inclusion in merge commands

- Add logic to correctly propagate submodule PR inclusion/exclusion in `rmerge`
- Add logic to read `--depends-on user/repo#pr` PR comments with travis-merge
- Add logic to read `-Iuser/repo#pr` in `scc merge` command
- Add unit tests for the submodule PR parsing
- Add `--info` argument to `TravisMerge` command for testing/debugging

Testing scenarios (openmicroscopy.git):
- `scc merge` can be tested with

 ```
python ~/code/scc/scc.py merge dev_4_4 --info -Dnone -Iopenmicroscopy/bioformats#523 -Iopenmicroscopy/ome-documentation#346 -I#982
```

- `scc travis-merge` can be tested with

 ```
TRAVIS_PULL_REQUEST=1200 python ~/code/scc/scc.py travis-merge --info
```