Custom image selection and add dragging controls

This pull request does three things, despite the title only mentioning two:
1. allow the user to drag along the grid to draw; drawing over an already filled tile darkens it
2. allow the user to select their own image to be converted into grid form
3. move the javascript and css code around into separate files because more files mean business

Sample image that can be used: 
![untitled-2](https://cloud.githubusercontent.com/assets/2467473/3633942/7299da7c-0f07-11e4-957d-02187b11e335.png)

Sample output from a larger image:
![screen shot 2014-07-19 at 1 24 49 am](https://cloud.githubusercontent.com/assets/2467473/3633943/88ea7bd8-0f07-11e4-85ad-87e738e497d6.png)