AWS Lambda Provider

This PR introduces a provider for [AWS Lambda](http://aws.amazon.com/lambda/).

Right now you have to either upload a zip file in the UI, through the command line, or use their console editor in the browser to create a Lambda. Lambda's are just JavaScript, so I want to be able to use the pull request development flow on GitHub with TravisCI for CD of my Lambdas. I think that being able to continuously deploy Lambdas from TravisCI could help grow an ecosystem of community Lambdas. At least that's an idea.. :)

The `lambda` provider is functional in its current state and has 100% test coverage. That said, there might be some issues that we should discuss if you even want to include it in the first place. Please let me know if there are any fundamental concerns with exposing this provider. Lambda is still in preview, but wanted to get the conversation started ahead of time.
I think context.env is better, it allows isolating in tests, for instance.