Fixed the installation instructions in the README for keybinder

[ValentijnvdBeek]

Fix issues #7 and #4 by adding instructions on how to install they keybinder dependency.

It also adds some extra instructions for installing the right packages on Ubuntu, using the --user flag to avoid having to use superuser unnecessarily, links to the dependencies, a warning about the package name and a few tweaks to the virtualenv.

Since keybinder can only be installed using a package manager it is impossible to install it in a virtual environment so you need to pass the --system-site-packages so it symlinks all the packages on your system to that virtual environment. I would also recommend installing the PyGObject in this way too since doing it via PyPy requires compiling and dev libraries while most people have it installed on their computer already.

[and3rson]

Hey, thanks for the information! I find this very helpful.

Just a small request: can you please fix the typo in bash command section (letter "C") in your changes?

[ValentijnvdBeek]

You're welcome, I just fixed the typo.

[and3rson]

Thanks a lot!