Python Framework build on OS X, Clawpack example

This adds the ability to install Python as a framework.
This is needed for interactive plots, but is also somewhat fragile.
For example, the framework install breaks the current scipy build,
so the two are currently incompatible until somebody has time to fix it.

@ketch / @rjleveque / @mandli - If this is interesting or useful to you please test it out and let me know what (besides SciPy is broken).  There's a new clawpack_dev.Darwin.yaml file in examples that you can use here by doing something like:

```
# or just fetch if you've got a repository already
git clone https://github.com/hashdist/hashstack
cd hashstack
git checkout ahmadia/python-framework
cp examples/clawpack_dev.Darwin.yaml .
vi clawpack_dev.Darwin.yaml
# I assume you've got hashdist on your path
../hashdist/bin/hit build -v clawpack_dev.Darwin.yaml
export PATH=${PWD}/clawpack_dev/bin:${PATH}
hash -r
cd ${CLAWPACK}/pyclaw/examples/advection_1d
python advection_1d.py iplot=1
```