[WIP] Add Deployments API support

## Add support for deployments API

This pull request includes the following functions:
- Support for preview APIs at the client level
- List/Create Deployments
- List/Create Deployment Statuses
there's not just one "preview media type".  GitHub assigns custom media types for different features, so a given application may need to use any number of media types depending on the methods they call.  So this should be renamed to more accurately identify what this is a preview for (for example, `mediaTypeMembershipPreview` below for the new membership APIs).  Like the membership type, also add a comment that links to the blog post describing the preview (https://developer.github.com/changes/2014-01-09-preview-the-new-deployments-api/ in your case)

That also means you don't want to set the accept header globally like you've done below in this file.  Instead, it should just be automatically added inside the methods that need it, like [here](https://github.com/google/go-github/blob/56f6b12f8ce99c254a30f48d87d8bb28024a9e4e/github/orgs_members.go#L165-L166).