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
from being run through the jshint through an "exclude" configuration.

Example configuration:

```yaml
java_script:
  enabled: true
  exclude:
    - "app/assets/javascripts/*.js"
```
Move javascript exclusion configuration

Moves javascript file exclusion configuration out of .hound.yml into the
language specific config.
Ignore JS files during lint based off jshintignore
Don't couple jshint_ignored_files to jshint

Renamed to `ignored_javascript_files`