Help tab

* Organize includes into subdirectory
* Move functions, actions to their own PHP files
* Add @Michelf's Markdown and MarkdownExtra
* Add Help Tab

**Help Tab**
This deserves some explanation: This is a tab that will appear next to the "Add Media" button in WordPress. It pops up a ThickBox showing the README.md documentation.

I think that our target audience with these shortcodes is slightly more advanced than your average end-user. As such I think showing the documentation is a really good way to educate and inform without cluttering up TinyMCE with tons of buttons.

Here's what the tab looks like:
![screen shot 2013-09-14 at 7 55 03 am](https://f.cloud.github.com/assets/2457670/1149411/6b4a4fea-1eda-11e3-9136-423282bd4d0c.png)

And here's the popup it displays:
![screen shot 2013-09-16 at 10 20 33 am](https://f.cloud.github.com/assets/2457670/1149458/3412f49a-1edb-11e3-8360-016d9c9e0d11.png)