react-flux-servant: Allow react-flux stores to send requests to a servant server

This project integrates react-flux with servant. There is a small example in the haddock documentation. To depend on this module, I add it to the extra-deps in my project's stack.yaml. See the react-flux documentation for a full example stack.yaml.

At the moment the code works well and is straightforward to integrate into a react-flux store; I am using it in production. Having said that, the current API requires you to create two store actions for each request and it is somewhat tedious to write all this boilerplate. At some point, I would like to build a better API, perhaps one that takes better advantage of the type-level Servant.API definitions to drastically reduce the boilerplate. Unfortunately, since it currently works (and even works well, the existing API is good), due to time constraints it might be awhile before I get around to thinking about better APIs. If you have any ideas for a better API, even if you don't have any code, please open an issue!