Revamp module initialization

Right now there are two ways to initialize Composer Manager:
- The init.sh script
- The drush composer-manager-init command.
Both call the same code.

Problems:
- There are two ways of doing the same thing, complicating documentation needlessly.
- The drush composer-manager-init command is sometimes not recognized, because the module was just downloaded.
Requires a clear of the drush cache.
- The init.sh script is not a shell script, it is a PHP script. We followed core's example, but it's stupid and requires us to remind
people to run the script with php.

Solution:
- Remove drush composer-manager-init
- Rename init.sh to init.php