Changing Structure. Service should be inside the Compute namespace.

As discussed on #1 i've refactored the structure to keep the service inside the compute namespace.
I have added also a file to properly load the lib when requiring the project as expected.
I agree with you @fernandes . But right now i'm more worried about the path of the files being kind of messy.
As an example, the [core](https://github.com/fog/fog-xenserver/blob/master/lib/fog/xenserver/core.rb) file: it doesn't define a class named `Core`. :astonished:

In a later pull request wen can take care of this, adding the new constant name, and deprecating the old one, giving the users an easier way to migrate to [fog-xenserver](https://github.com/fog/fog-xenserver) :smile: