Use `.travis.yml` notifications for `.travis.osx.yml`

This copies over the notifications settings of the main
`.travis.yml` to `.travis.osx.yml` so that we can maintain
them centrally.

The change also tweaks the commit message to aid in quick
linking / correspondence of a pantsbuild/pants-for-travis-osx-ci
commit to a pantsbuild/pants commit.

Testing Done:
Ad hoc:
```bash
$ venv_setup pyyaml PyYAML && source ~/.pythons/venvs/pyyaml/bin/activate
(pyyaml)$ python << EOF
> import yaml
> with open('.travis.yml') as fp:
>   config = yaml.safe_load(fp)
> with open('.travis.osx.yml') as fp:
>   config_new = yaml.safe_load(fp)
> config_new['notifications'] = config['notifications']
> with open('.travis.new.yml', 'w') as fp:
>   yaml.safe_dump(config_new, fp)
> EOF
(pyyaml)$ cat .travis.new.yml
language: objective-c
notifications:
  email: [john.sirois@gmail.com, lhosken@twitter.com, zundel@squareup.com, benjyw@gmail.com,
    areitz@twitter.com]
script: 'sw_vers

  python --version

  java -version

  ./build-support/bin/ci.sh -d

  '
```

Bugs closed: 601

Reviewed at https://rbcommons.com/s/twitter/r/1067/
remove some more redundant docstrings

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/35376388

Reviewed at https://rbcommons.com/s/twitter/r/1035/
new "sitegen" goal in internal backend

Defines a new "sitegen" goal. It's the start of a static site generator for Pants docs.

We don't want to ship this goal to all Pants users.
We do want to use it in the Pants repo.
So set up a new dir src/python/internal_backend/
It doesn't get pex'd up with src/python/pants code.
But can be pulled in by adding backends to pants.ini
(e.g., build-support/bin/publish_docs_new.sh creates pants.ini.sitegen by appending a backends to pants.ini)

So far this goal looks for a site description config'd in a json file,
and based on that copies some .html files
(fixing up links between them to be OK in spite of relative-path changes)
and wraps them in a potentially-pretty .html template and stylesheet.
That ain't everything it should do; but it's enough to get an idea of code structure.

To invoke this code,
./build-support/bin/publish_docs_new.sh

Testing Done:
added tests/python/internal_backend_test:: to ci.sh.
https://travis-ci.org/pantsbuild/pants/builds/35997435

Reviewed at https://rbcommons.com/s/twitter/r/1030/
Merge branch 'master' into areitz/add_publish_plugins