Integrating reqwest's `async` module as a rust module

Created by: srenauld

This commit contains modifications to the Rust codegen module in order to be able to generate futures-aware, threadsafe reqwest bindings from an openAPI template, in order to be able to take advantage of non-blocking API calls.

The changes are as follows:

• Added a library reqwestFutures
• Dependency changes in Cargo.toml (futures is now a dependency for reqwest when using this library
• Due to the requirement that all futures should be Send, the reference-counting pointer is now an Arc
• Templates, evidently. They're very similar to the reqwest module itself, as the API from the sync and async client are virtually the same

Due to the upcoming changes regarding async/await, I took the deliberate decision of calling the library reqwestFutures rather than reqwestAsync. When Rust 1.39 lands and those features become stable, a conversion tool from one to the other should be easy to build.

Rust committee members are @frol @farcaller @bjgill @richardwhiuk - since this is my first PR here, I guess mentioning them should be good enough as a CC?