remove PIL, add Pillow in ec2 init script

This PR modifies the ec2 cluster launch script to first install pip on all workers (in addition to the master, where it was already being installed). It then uninstalls the existing python-imaging package, which installs PIL 1.1.6. This uninstall technically breaks a lot of other packages that list python-imaging as a dependency. We then install libtiff, itself an optional but suggested dependency for PIL / Pillow, then use pip to install Pillow on the master and all workers.

The upshot is that we replace PIL v 1.1.6 with Pillow 2.6.1. This fixes #51, and probably puts us in a better place to address future tif-related issues, since we're now using an up-to-date package on ec2.