Update App Header

https://trello.com/c/5R0ophhT
Disable GuardClause cop
Disable action filter check for Rails
Disable ClassAndModuleChildren cop
Disable EachWithObject cop
Update `line_number` to `patch_position` where it makes sense.
Update readme for development installation.
Document testing enviroment setup
Update style guide link in README
Add config option to display cop names in comments

* add `ShowCopNames` config option to show cop names in GitHub comments
  * turned off by default
  * to enable set `ShowCopNames: true` in `.hound.yml`
Fix erroneous logic to disallow duplicate comments

* This commit fixes two logic errors in be2dd34178
* In `Commenter#previous_line_comments`, compare position of existing
  comment to line patch position rather than line number
* In `CommentPolicy#violation_not_previously_reported?`, compare new
  violation message to body content of existing comments rather than the comment
  objects
* https://trello.com/c/ha2hgFj2/283-avoid-commenting-a-second-time-after-a-force-push
Upgrade RuboCop

* memoize StyleGuide#config result
* add tests for old-style syntax
* call `add_missing_namespaces` on override (custom) config to ensure that
  merging is handled properly

https://trello.com/c/K7dSqe3k
Disable EndOfLine cop in tests

This cop breaks when dealing with strings (instead of files).

Here's a RuboCop PR to address the issue:
https://github.com/bbatsov/rubocop/pull/1203
Compare violation messages before commenting

* This commit ensures that:
  - to determine whether a comment is permitted, we check existing comments'
    `original_position` rather than `position`, which is nil after force pushing
  - violation messages in existing comments are split and checked
    individually, rather than as one large string with line breaks
Add support for checking CoffeeScript

Adds support for checking CoffeeScript source files for style violations using
CoffeeLint.
Gracefully ignore files of an unknown language

Files of all languages will be passed to an instance of `StyleChecker`, and it
shouldn't break because it doesn't know how to check it for style violations.
Style fixes
Rename NullStyleGuide -> UnknownStyleGuide

The lets us get rid of the `begin..rescue..end` clause.