Rename methods for adding comments

* I have a feature branch that implements issue commenting. This commit
  renames methods in `PullRequest` and `GithubApi` in anticipation of the
  changes on my branch.
* Rename `PullRequest#add_comment` to `PullRequest#comment_on_violation`
* Rename `GithubApi#add_comment` to `GithubApi#add_pull_request_comment`
Allow a user to exclude certain JavaScript files

Allows a user to configure hound to ignore specified JavaScript files.
This follows the "jshintignore" convention. If a .jshintignore file is
found and the user has not overwritten the path in the hound
configuration, it will automatically be used.

Otherwise, a user can specify the path to a file which specifies which
files to exlude during linting.

Example configuration:

```yaml
java_script:
  enabled: true
  ignore_file: ".javascript_ignore"
```

.javascript_ignore
```
app/assets/javascripts/**.js
``