std::future compatibility

This release migrates to the std::future version of hyper thus enabling the use of async/await syntax.

The code should be backward compatible (given the new Future, that is). You can probably face lifetime issues (especially in the Cosmos SDK) that can be overcome avoiding references altogether.
I will probably streamline the Cosmos crate in the near future to make it more similar to the other ones.

Fixes #170, #169, #168, #167, #166, #165, #164, #163 and #140.

It completes the milestone #2.