Javadoc cleanup (#421)

The changes were largely driven by Java 8's enhanced javadoc linting.

* Only use header tags if the previous number has been used in the page (start at h3)
* Remove empty p tags
* Remove improper p end tags
* Fix some dead link references
* Correct unescaped special HTML characters in code examples (less than, greater than, ampersand)
* Fix a couple bad pre tag attributes
* Fix an improperly nested list
* Fill in some missing @param and @return docs
Update api links to include trailing slash (#421)

Java 8 seems to complain about redirects for these URLs
Fix a MethodSelectionException for Java 8 (#421)

Java 8 added a new signature for remove to Map that takes two arguments.
Without the arguments specified, it can't figure out which one, and throws a MethodSelectionException.
Fix a javadoc link
Don't run site tests under Java 8 for now (#421)
Convert TestHttpClient to be based on the ratpack httpclient

Add autoclosable to the TestHttpClient, DefaultTestHttpClient, and BlockingHttpClient

Change TestHttpClient to be based around actions configuring the request.
Set timeout on blocking httpclient latch so bad request specs don't hang the testing.
Core tests passing

Added cookie support to the DefaultTestHttpClient
RequestSpec javadoc work.
Bump Contributor to Active Member
ReceivedResponse javadoc start.