Added provider for CloudSigma

Hi,

I created a provider for CloudSigma. Currently only the basic functionality is covered with tests (CRUD for volumes and servers, and attaching networks to servers).

I can provide some free resources for testing against real infrastructure. The tests will also work with the resources available from a trial account. Accounts can be created at https://lvs.cloudsigma.com/ui/ (I am mentioning it because there is also a legacy system, and the provider is for the new system).

Any feedback will be appreciated.