WebRunnerTests now working, thanks to adding FakeHttpAsyncClient.
Fixed some of the failing tests in RequestRunnerTest and removed some others.

The removed tests were very complex and not core functionality. They'll
most likely also be easier to implement on a higher abstraction level
(e.g. in WebRunnerTest).
Cleanup (removal of debug message and unused imports).
Extracted anonymous class to new name: WebRunnerByteConsumer
Merge branch 'async-web-runner' of https://github.com/SmartBear/loadui into async-web-runner
Merge branch 'async-web-runner' into async-web-runner-test-fixes
Misc small cleanups.
Refactored HttpWebResponse to make it immutable.
No longer reusing WebRunnerStatsSender.

I asked Renato about this. He didn't know what this was for and
suggested that I removed it.
WebRunnerStatsSender.resourceToVariableGroup is now thread-safe.
RequestRunner.pageRequest and RequestRunner.assetRequest is now thread-safe.
WebRunnerStatsSender now supports adding resources sequentially again.

I mistakenly broke it earlier in this PR.
Merge pull request #56 from SmartBear/web-runner-refactoring

Web runner refactoring