cask-filename `Cask` → `Caskfile`

**Note: breaking change.** There isn't a WIP branch open so PR against master for discussion.

Changes `cask-filename` from `Cask` to `Caskfile` to align with the convention used by other notable tools.

* [Make](https://www.gnu.org/software/make/) and `Makefile`
* [Rake](https://github.com/jimweirich/rake) and `Rakefile`
* [Bundler](http://bundler.io) and `Gemfile`
* [Boxen](https://boxen.github.com) and `Puppetfile`
* [CocoaPods](http://cocoapods.org) and `Podfile`

`make test` passes for me locally. Also tested with this emacs config `.emacs.d/init.el`:

```emacs
;; .emacs.d/init.el                                                                                                                                                              
(require 'cask "~/src/cask/cask/cask.el")                                                                                                                                        
(cask-initialize)                                                                                                                                                                
```