[WIP] Support providing phar package `apigen.phar` (fixes #237)

This PR adds support for running ApiGen as a single self-contained `apigen.phar` archive with all dependencies included for easier distribution. For example, the resulting user command might look like this:
`./apigen.phar --source ./src/ --destination build/doc`

Packaging the whole project into a phar was actually quite easy by using [clue/phar-composer](https://github.com/clue/phar-composer):
`./phar-composer.phar build ~/workspace/apigen`

However, ApiGen (and unfortunately also some of its dependencies) need a few minor patches, as `realpath()` does not work within phar archives:

- https://bugs.php.net/bug.php?id=52769
- https://github.com/zendframework/zf2/pull/297
- http://framework.zend.com/issues/browse/ZF-11115

Current status:

- [ ] Wrap *all* calls to `realpath()`
- [ ] Update referenced dependencies to also wrap calls to `realpath()`
- [x] Provide a sample `apigen.phar`: [Temporary download link](http://www.lueck.tv/temporary/apigen.phar)

This PR is a work-in-progress, but I felt it's useful to get feedback at this stage already.