Pr require command

This add support for 'require' command and add exec to the factory so we could do something like this from hiera .yaml file

composer::projects:
  devlib/devlib:
    project_name: devlib/devlib
    target_dir: '/home/user/my-libs/devlib/devlib/'
composer::execs:
  phpqa:
    user: vagrant
    cmd: 'require'
    cwd: '/home/vagrant/.composer/'
    prefer_dist: true
    packages: 
      - 'phpunit/phpunit=3.7.*'
      - 'codesniffer/codesniffer=3.7.*'
      - 'cyclomatic-complexity=3.7.*'
  another-load-of-libraries:
    user: vagrant
    cmd: 'require'
    cwd: '/home/vagrant/.composer/'
    prefer_dist: true
    packages: 
      - 'whatever-1
      - 'whatever-2'
      - 'whatever-3'