As I was looking for API reference for TFS Release Management, I came across this useful link that I wanted to pass along. Visual Studio Online REST API Reference page (https://www.visualstudio.com/en-us/integrate/api/overview) includes API reference for TFS components like build, work item tracking, version control/GIT, load test, test management, shared services, etc. Basically, it includes reference for all existing API hooks for TFS/VSO.
I have also found this blog post (http://blogs.msdn.com/b/aseemb/archive/2014/12/23/how-to-program-against-release-management.aspx), which includes the API reference for TFS release management. It includes sample code that allows you to create a user, environment, release path, release definition, etc. Please note that release management (like build) is changing quite a bit in TFS 2015, so it's safe to assume that the API for build and release management will also be changing. Visual Studio Online REST API Reference page should have the latest changes, so just use it as a reference :)
If you haven't already seen it, look at the information on getting started with these APIs.