Add phing-based build system to make it easier for contributors to build, install and run tests

Any build system like Phing (including: make, Grunt, Gulp, SCons, Ant, etc) would be extremely helpful for Panopoly contributors.

Basically, it'd allow contributors to create a configuration file that tells the build system about their environment and their preferences and then run simple commands, like "phing make-profile", "phing install" or "phing test", to easily perform common contributor tasks, as well as do any necessary setup and run dependent tasks when necessary.

For example, let's say you just checked Panopoly out of Git and hadn't done any setup yet. By running "phing test", it would see that you haven't yet built the profile and run "phing make-profile", see that you hadn't yet installed Panopoly and run "phing install" to install a Panopoly site via drush, as well as download Behat and it's dependencies via composer, download and start selenium and finally run the Behat tests.

This would allow contributors to get started really, really quickly without needing to do any of those things manually (or even know how to do those things).

I think Phing (vs Grunt, Gulp, etc) is right choice for a number of reasons:

1. It's built on PHP, so if you're running Drupal, you already have it. You don't have to install another tool chain like Node, Python, or Ruby
2. It's built on PHP, so to extend it, you write PHP. If you're a Panopoly contributor, you're more likely to know PHP.
3. You can install it via composer, which is also how you install Drush, Symfony, Behat and Drupal 8

So, while Phing isn't the new hotness (like Grunt or Gulp) and it uses a Ant-inspired XML files to define targets (ick!), I think it's the right choice for a Panopoly, because our highest priority for this functionality is allowing contributors to start contributing with the lowest amount of friction possible. Yes, "go install Node" is more friction than I want to introduce. :-)

TODO

1. Switch 'phing' install to composer. The README still suggests installing with pear - this is left over from a much older iteration! We should include a composer.json/lock in Panopoly to install phing and any other build dependencies.
2. Implement 'install' target
3. Implement 'test' target