Refactor spectator metrics

__Problem__

Currently the listener does not expose metrics to specific events but instead does internal calculations and expose custom gauges. This is limiting as it gives less information.

__Modification__

Modified the metrics to represent the actual events.

__Result__

More flexible insights.

`RequestN` reduces with completed subscribers.

#### Problem

Re-calculation of max `requestN` per subscriber did not take into account removed subscribers correctly. This causes the `requestN` value to decrease over time when subscribers are completed.

#### Modification

- Modified `recalculateMaxPerSubscriber` to take old and new subscriber count instead of trying to determine the correct value from the current subscriber queue size.
- `subscribers.remove()` does not happen on unsubscribe but only from the task that recalculates the max `requestN` values. This also coalesces multiple removes into a single task run.

#### Result

Consistent `requestN` values with new subscribes and completions.

Escape html special characters in default error response generator.

Upgrade netty to 4.1.5.Final (ReactiveX#547)

Upgrade netty to 4.1.5-Final (ReactiveX#546)

Removed unused code (ReactiveX#540)

Cleanup for ReactiveX#525, removing unused methods and classes.

Update README.md

Disable `LocalEchoTest` (ReactiveX#519)

Travis does not seem to be happy about this test, although it passed for the PR.
Disabling this for now as this is not really critical functionality.

Fix project release (ReactiveX#508)

Fixes ReactiveX#474 (disable auto release leaks buffers when content …

…not subscribed) (ReactiveX#497)

When auto-release it turned off and the HTTP content is not subscribed, the buffers which are disposed from `UnicastContentSubject` are not released (which otherwise would have been released by the subscriber).

This change properly release the disposed buffers when auto-release is turned off.