Support lowercased phakefile filenames.

Support lowercased runfile names to be recognized as well. Example:

    phakefile
    phakefile.php

Tests for proper file recognition added aside.

PHPUnits bootstrap now uses composers autoloader in order to benefit
from autoloading for test classes as well. The composer.json file
reflects PHPUnit and vfsStream dependencies (as _dev_).

Please note: Usage of 5.4 short array syntax in tests! Consequence: 
The _tests_ require PHP 5.4. I'm not sure about the compatibility 
strategy of `phake` with regard to PHP versions, but I just wanted to
give it a go in tests. If strict 5.3 compliance is desired, let me know.

Feel free to review and comment. Thank you.