Add support for Parallels Desktop for Mac [GH-223]

This adds support for building images for [Parallels Desktop 9 for Mac](http://www.parallels.com/products/desktop/). [GH-223]

It adds two builders:
  - *parallels-iso*: Which creates an new image from a bootable ISO.
  - *parallels-pvm*: Which creates an new image from a existing image in the _pvm_ format.

It also adds support for _Parallels Vagrant_ box creation, that can be used with the [Vagrant Parallels plugin](https://github.com/Parallels/vagrant-parallels).

Thanks to @YoungSang for the original _Vagrant post-processor_ adaptation to Parallels. Also thanks @legal90 for testing and support.