Adding fix to allow gp.recipe.phantomjs to honor relative-paths buildout option

In buildout, there's an options "relative-paths" that allows the built-out files to be moved to a different directory:
https://pypi.python.org/pypi/zc.buildout/2.2.1#relative-paths

This is a fix to make sure gp.recipe.phantomjs honors relative pathing.

I also added a few tests to unit test my new functionality. Also added a travis.yml file to check my changes:
https://travis-ci.org/toumorokoshi/gp.recipe.phantomjs.

I wasn't sure how to run tests.py, so I couldn't add it to the travis.yml file.

I'm sorry about the big patch, but this is a significant change and wanted to ensure my code worked as desired.