[WIP] Add support for getting config from environment variables

I've been looking at ways to make pkgcloud easier to integrate, and one of the options is using environment variables.

I've taken a first pass at one idea, using a per-provider prefix to get the values from the environment. Here's a contrived example:

```
$ PKGCLOUD_RACKSPACE_USERNAME=myusername PKGCLOUD_RACKSPACE_APIKEY=1234asdf PKGCLOUD_RACKSPACE_REGION=iad node my-app.js
```

I wanted to open the PR to start discussion...