Convert Docs from HTML to Markdown: Usage & Installation

First batch of changes for #1180.

So far I converted the usage and installation sections. Furthermore I fixed a small bug with a broken if statement that resulted in the pagination not being shown (like [here](http://docs.ghost.org/installation/troubleshooting/)).

This may still contain a few mistakes that I haven't found, but I guess getting other people to read it is the best way to find them.