Cleans up the repo to make it work as a gem

Hey there,

I'm working on integrating this into the Bauer app and wanted to get this gem working and basically follow Ruby conventions. Now people should be able to include it in their Gemfile with:

```
gem 'play_captcha', github: 'r38y/playcaptcharubygem', branch: 'make-it-a-gem'
```

And have it load automatically. This is usually the minimum to have it work as a gem in a Rails app. Releasing it to RubyGems is a bonus. I had to rename a bunch of stuff because the constant was `PlayCaptcha` and Rails/Ruby tends to load things automatically when named that if the file is `play_captcha.rb`. 

I removed the tests because they didn't match the code at all and were pretty much useless. 

I haven't updated the docs yet because I wanted to wait to see what you guys thought. If you accept it the above line would change to:

```
gem 'play_captcha', github: 'FutureAdLabs/playcaptcharubygem'
```

I would consider renaming the repo name from `playcaptcharubygem` to `play_captcha` to follow all the conventions. That isn't strictly required though.