Added the DigitalOcean provider

Added the [DigitalOcean](https://www.digitalocean.com/) provider. Implemented portable interfaces are:

* ComputeService
* ImageExtension

All unit and live tests are passing (full test log [here](https://gist.github.com/nacx/8558488)):

    Unit tests: Tests run: 69, Failures: 0, Errors: 0, Skipped: 0
    Live tests: Tests run: 52, Failures: 0, Errors: 0, Skipped: 0