When Composer isn't globally installed, the `composer global` call results in an error. This checks to see if Composer is available before making the call. When Composer isn't available, it will just manually set the directories.
Add the proper config bin directory to `PATH` instead of the previously (incorrect) fixed `~/.composer/vendor/bin`. Nowadays the right config dir is `~/.config/composer/vendor/bin`.
This is because we can't be sure which arguments require a json file
and which don't, so the `else' statement will always be difficult to
check for correctness.
Use $_comp_command1 for command list generation instead of 'composer':
`$_comp_command1` contains the command written in the command line.
This allows us to have a different composer name
This plugin currently doesn't autocomplete composer commands if there is no composer.json in the current dir. However, the commands create-project, init, search, selfupdate and show are still useful without a composer.json!
