User installed plugins

We want the plugin system in elisa to be as easy to use as bzr's: to install and activate a new plugin, just drop a python egg in ~/.elisa-0.5/plugins, run elisa, and there you go!

For this to work, we need to use setuptools correctly. Currently what is done is that elisa's launcher fiddles with the PYTHONPATH, invoking some hard-to-debug-and-hard-to-maintain black magic. We want to get rid of that, implementing a clean path lookup system.