Load themes via AJAX from a .json file

For #9.

This PR makes index.html load the currently available themes via AJAX from the newly added `themes.json` file, which can easily be edited.

_Advantages_:
* Changes are easy, easy maintenance
* There's no more need to edit index.html in order to add themes there
* New data keys can easily be added and displayed on the website

_Disadvantages_:
* Loading times (not too long)
* Worse SEO

So basically, this PR removes all the old, static content about themes and puts in a placeholder where the themes will be displayed later on. At first, a newly added spinner will show up and will soon be replaced by the actual content.

Further changes:
* .gitignore added (only some files which often get into git by accident)
* .brackets.json added for some project preferences
* README: Make the link to the actual site look nicer
* Some indendation stuff

[Demo](https://67cf2aa7ef478c0cd033d8ccdcb334d1a683efb8.googledrive.com/host/0B986Uw1nLGY7N2NUVHlCd3pSOFE/index.html)