Converted PhpSpec to a single-command application

This makes running phpspec simpler: it is just 'phpspec' now instead of 'phpspec run'. The majority of commenters in https://github.com/phpspec/phpspec/issues/238 seemed to agree that it is better.

I tend to think that a single-command app is far more logical in the case of PhpSpec.

The second commit adds a BC layer, so that ``phpspec run`` still runs the full suite instead of an empty one, to avoid breaking the testing of libraries using PhpSpec on their CI server. It outputs a message instead to tell users to upgrade their command.
this BC layer should be removed at some point in the future.