Softlayer

I've got some time budgeted to focus on SoftLayer tooling.  Here's an initial pull request adding a SoftLayer provider that follows the new modular approach (modeled pretty closely after the existing Brightbox patterns).

I'm pushing this up now looking for early feedback.

Next steps are: 
* `Compute` 
  * add more requests
  * models/collections
* `Storage`
* `Image`
* `DNS`
* `Network`

Thx!