Akka HTTP backend support for chunked responses

Still need to implement support for trailing headers.
Use normal string literal to fix Sublime's syntax highlighting

Sublime struggles when it sees four quotes in a row.
Merge pull request #3743 from richdougherty/akka-http-integration-pt3

Akka HTTP backend support for chunked responses
Convert integration tests to run against both Akka HTTP and Netty

Not all tests are currently working with Akka HTTP. Failing tests
are marked with pendingUntilAkkaHttpFixed or
skipUntilAkkaHttpFixed so that they don't cause test failures.
Merge pull request #3744 from richdougherty/akka-http-integration-tests

Convert integration tests to run against both Akka HTTP and Netty