Replace 'btn-inverse' with 'btn-default' to fix display issue.

### What

With the exception of the root page (which is using the "homepage" layout), it seemed that all pages were using the "application" layout, which has buttons using the class `btn-inverse`. This bootstrap class made buttons difficult to read, as seen below:

Class of `btn-default`: 
![image](https://cloud.githubusercontent.com/assets/4978418/5194831/887fdd60-74e1-11e4-860d-8e2403992c0f.png)

Class of `btn-inverse`:
![image](https://cloud.githubusercontent.com/assets/4978418/5194854/c01e03a0-74e1-11e4-925a-741334c93d0f.png)

### How

I simply changed the button class used in the application layout file, which every page but the home page is using.

### Why

In addition to the improved consistency and readability, it appears that the `btn-inverse` class has been [removed from Bootstrap 3](http://getbootstrap.com/migration/), and some guides suggested [replacing it with default](http://code.divshot.com/bootstrap3_upgrader/).