Make app/generator/generate.php more portable

The former way the php executable was searched just in /opt/local/bin.

Now, it is searching all user's environment for that.

Source: http://www.cyberciti.biz/tips/finding-bash-perl-python-portably-using-env.html
Signed-off-by: Rogerio Prado de Jesus <rogeriopradoj@gmail.com>
Merge pull request #18 from rogeriopradoj/portabilize-generator

Make app/generator/generate.php more portable