update button classes to reduce file size & increase reusability

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 it also reduced the file size by 2 bytes
add .navbar .btn selector

after my previous commit I realized that this would need to be included as well.

still keeps the modularity of the .btn class, but ends up making the minified/gzipped file back to 980 bytes
navbar & headings update
Merge pull request #9 from scottaohara/update_btn_classes

update button classes to reduce file size & increase reusability