Add Phing build script to automate releases

Creating a release involves some tedious manual work that could be streamlined. I have tried to do so using [Phing](http://www.phing.info/docs/guide/stable/).

Using this creating a release (e.g. 2.3.4) should be reduced to:

* `./vendor/bin/phing -Dversion=2.3.4`
* Potentially adjusting `CHANGES`. Commit messages are not always 100% readable
* `git push origin master 2.3,4`

This could be extended with tasks such as:

* Updating gh-pages
* Adding new contributors to README.md
* Building a phar file (as suggsted in #53)

This needs to be updated if #52 is merged.