Add composer_cmd as parameter for composer tasks

_I've made this pull request yesterday but today I noticed that I'd created a branch from `master`. This one is created for `develop` branch._

By default `composer` tasks execute composer command with `php composer.phar` in project path. It's a bit problematic, when composer is installed globally or in custom path. 
This PR includes customizable `composer_cmd`, so calling `composer` tasks is as simple as:
```
tasks:
  pre-deploy:
    - composer/install: {composer_cmd: 'composer'}
```

I've also changed all occurences of `composer_path` to `composer_cmd` because previous `composer_path` was not exactly the path where composer is but the command to call to run composer (_Boy Scout Rule!_)
Tested and working. Feel free to review!