Install from pip feature

Made this small modification to allow for installation from pip
Ah, sorry, should have been more explicit. These packages should use the previous method, `ensure_packages` but the `diamond` package should be a package resource.

The rationale here is that this module should own the management of diamond, if you have a diamond package declared elsewhere this module is unlikely to work, so best to fail early. However, thinks like pip and python-dev could well be included elsewhere, this module shouldn't _own_ those. Hence ensure_packages.