Allow running guake without installing first

With these changes, I am able to run (and test) Guake from
```
$ gconftool-2 --install-schema-file=data/guake.schemas
$ rm -f guake/guake.pyc
$ PYTHONPATH=. python guake/main.py
```
as an *unprivileged* user.

A quick summary of the changes done/needed would be:
* turn the `src` folder into a Python package `guake`,
* rename `guake.py` to something else (`main.py`) since the same name for package and child module is asking for trouble,
* add a symlink so that `globalhotkeys.so` is actually found, and
* make `globals.py(.in)` detect if Guake is run from Git or a system installation.

I hope you like this direction as it will enable more people to test fixes to Guake easily.