update button classes to reduce file size & increase reusability

Hi there,

I was hoping I could contribute the following to your project.  

In an effort to cut down on file size & make the button classes more modular, I reduced the selectors of the buttons to no longer be attached to the <button> element.

now .btn-a, b, c, & small can be combined with a generic .btn class (which has the same styles as <button> and this class combo can be added to <a> elements, inputs with a type of submit or anything that one would like to look like a button.

I used terminal to run a gzip compression on the minified file and these edits reduced the file size to 978 bytes (ran a compression in the same way on the original, which clocked in at 980).