Simplified the gen.py file. It now only searchs for an assets.json file in the project root.
The gen.py file now adds it's location to the sys.path so that extensions can import it.

Soon convenience functions will be declared in this file.
Two new generators that recursively loop through the contents of a directory.
The static type is now fully implemented.

It removes files that are not specified and only updates relevant ones
when they are older than the source!
We have no use for the __pycache__ folder so it's now in .gitignore.
Moved the other types out of the gen folder, finally.
New system.

The transformation between assets and output files are now handled by a
hierarchy of classes called content providers. They collect input and
then install their files, using an arbitrary transformation, or none at
all.
The notify_* methods of the Environment class should be private (marked as implementation).
The absolute path added to _sources in the static content provider should be normalized.

Just to avoid /./ and such in the middle of the path, etc.
Added a Jinja2 content provider!