Make base_dir configureable.

For my Rails apps, I want to be able to run phantomjs out of the vendor
directory. This change makes that very easy:

1) Create an initializer; config/initializers/phantomjs.rb
2) Set the base_dir:
  Phantomjs.base_dir = Rails.root.join('vendor', 'phantomjs')
Merge pull request #28 from kofno/feature/configure-base-dir

Make base_dir configureable.