Add Dotenv.ensure

I started writing in some checks into my app that ensured certain ENV variables were set when new developers (or myself on a new machine) clone the app, start working, then get vague errors about method_missing on nil. 

I know standard practice is to set a .env.example, but I'm also trying to think of a way to manage config requirements in-app based on certain contexts. For instance, my app may want to use AWS, thus needing AWS env vars, but not worry about setting Rackspace configs.

Future improvements could include descriptions of what these ENV variables are supposed to contain.

This may also enable the creation of a tool to analyze code for calls to Dotenv.ensure and generate a .env.example.