Improve command structure to simplify the maintenance of new commands

Each command should have the following functions:

* command_description: This gives a short description for the _phpbrew help_ command
* command_help: gives the long description for _phpbrew help commandName_
* command_run: runs the command itself

I adjusted the help command that it reads out the information of every command. I think that will give the commands more structure and keeps the help information of commands inside of the command scripts which will help with the maintenance and makes it easy to add new commands.