Enhance the execution harness to allow detecting if the execution completed without returning the promised value or propagating the error.
Error if a promise is used more than once.

Closes #440.
Add mention of the exec harness for promises.
Publish metrics using Reactive-Streams support
Restructure the header handling to allow full use of pre-rendered headers to avoid garbage creation.
Test fixes.
Use UTF8 to read typed data when no charset data is available.
Streamline the response handling to reduce the amount of garbage created per request.
Fix race in test.
Update benchmark app.
Remove redundant assignment.
Render a byte buf as http content.

Not doing this breaks keepalive support.
Add a FanOut publisher
Add the ability to cache and reuse promises.
Initial commit for streaming Hystrix metrics out as SSE (#346)
Reset Hystrix metrics before test run (#346)
Use Java 8 for the site.
Update Hystrix package info
Reduce the default Hystrix metric publishing to 2 seconds
Move to Java 8.

Closes #444.