Breaking Changes

Drop support for reqwest 0.9 (previously enabled via the reqwest-09 feature flag); only the (default) reqwest-010 feature flag is now supported
Migrate public API from http 0.1 to 0.2
Drop support for futures 0.1 and remove the futures-01 and futures-03 feature flags; only async/await is now supported (without requiring any feature flags)
Eliminate Async* traits and move the request_async methods to the underlying *Request structs
Return error types that implement std::error::Error instead of failure::Fail
Add Send and Sync bounds to verification functions to enable sharing across threads

NOTE: These changes track the unstable 4.x release for the oauth2 crate.

Other Changes