Add feign module

[jhalterman]

Add a module supporting Feign. This is pretty straightforward to do for sync executions.

For async executions, it seems like Feign's AsyncFeign class is still experimental, and doesn't play nicely with an InvocationHandler since the async code uses a ThreadLocal and throws a NPE if called by an InvocationHandler from a different thread. It may not be worth adding async support until things mature a bit.

[Tembrel]

Clarifying: https://github.com/OpenFeign/feign