consolidated sys.path alterations for more reliable boot sequence

the top-level __init__.py is now responsible for finding pyobjc
the nodebox script now sets PYTHONPATH correctly to allow the
subprocess invocation ot task.py to succeed
new nodebox.script module to let external scripts import *

- creates a context + canvas and adds all the drawing commands to the
script's namespace.
- includes an `export` context manager for writing images/anims
- also necessitated getting rid of the runloop dependencies in
export.py and cIO
added context managers for pdf pages & image sequences
trivium
added fonts() utility to find installed typefaces

tempted to add some of the other convenience functions from
http://nodebox.net/code/index.php/PyObjC as well...
made the controller layer a little smarter about image caching

also moved the fullscreen gui objects into views.py (where they belong)