Better support for Composer

Added a more info to composer.json that most libraries use. Including enforcing that the installing system have the mcrypt extension enabled: http://getcomposer.org/doc/02-libraries.md#platform-packages

Libraries should not version composer.lock. Read more here:
http://getcomposer.org/doc/02-libraries.md#lock-file
http://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file

Libraries also should not distribute a copy of the composer binary.

Also changed the phpunit binary to be installed in the project's bin dir. This is a little more conventional than digging into the vendor dir to get it.

While I didn't include it in this PR, there is still room for more improvement by leveraging Composer's autoloader instead of your library distributing its own: http://getcomposer.org/doc/01-basic-usage.md#autoloading