Added support for plugins.

I've added an easy way to load additional phases. This is required in #43 for splitting Soot into smaller pieces. The main idea is a new parameter that receives a configuration file where the plugins you want to load are mentioned. This allows us to have some kind of plugin ecosystem where you can easily test phases written by others.
I've written some documentation [here](http://blog.security-comprehension.org/?page_id=154).