Add composer_cmd as a parameter to composer commands

By default `composer/install` executes composer command with `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/install` task 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`.
Tested and working. Feel free to review!