Add install_executable, dump_autoload and self_update tasks

As discussed in #6 

3 new tasks

* `composer:install_executable`
* `composer:self_update`
* `composer:dump_autoload` (with composer_dump_autoload_flags option)

I have hesitated to add command mapping as I cannot see a clean way of working with either both a "managed" version of composer, or a user provided one - I think the user should be encouraged to do the command mapping themselves in deploy.rb?

If/when this gets merged it would be good to have a new tag ;)