Improve installation & instructions

- symlink executable to `/usr/local/bin/prax` in installation
- add instructions to recommend cloning to a proper location such as `/opt`, and how to autostart the server by placing `prax start` into `.profile` or `.bash_profile` (would've added this to installation too but alas debian has no standard way to autostart commands run as a user, except for modifying these login files directly. Actually, another way would be to have the init script scan all `/home` directories for `.prax/`, and whichever has those, start prax under that user. Maybe later)

With this in place, all that's left to get to an actual package is using something like [fpm](https://github.com/jordansissel/fpm) to put all the files in `/opt` and run `prax install` after the package has been installed.