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 (this is 50x7 pixels): ![untitled-2](https://cloud.githubusercontent.com/assets/2467473/3633942/7299da7c-0f07-11e4-957d-02187b11e335.png)

The output from that small image:
![screen shot 2014-07-19 at 1 45 04 am](https://cloud.githubusercontent.com/assets/2467473/3633947/eacad46a-0f07-11e4-9241-d604cd612a9d.png)

Sample output from a larger (regular sized) image:
![screen shot 2014-07-19 at 1 24 49 am](https://cloud.githubusercontent.com/assets/2467473/3633950/0ce0d63a-0f08-11e4-81b4-a6efb471bb9c.png)