Add support for railway-oriented OWIN

Status: Work in Progress

- [x] Provide immutable `Environment` updates (see commit 985ad5c)
- [x] Ensure `.With` can update existing keys (see tests in commit 1f1ed5c)
- [x] Copy new `Environment` to original `OwinEnv` (see tests in commit 1f1ed5c)
- [x] Copy response `Stream` asynchronously to the original output `Stream in `OwinEnv`
- [x] Adapt `OwinRailway` to `System.Net.Http` types in `SystemNetHttpAdapter`
- [ ] Add tests for `System.Net.Http`-based `OwinRailway`.

Links:
* http://fsharpforfunandprofit.com/posts/recipe-part2/
* http://www.slideshare.net/ScottWlaschin/railway-oriented-programming
* http://vimeo.com/97344498
Is this everything?