Simplify representation and make more usable

* drop all protocol methods but as-reponse
* charset conversion is done by the String representation
* broken gzip encoding is dropped
* use clojure.lang.Associative instead of APersistentMap
Refactor representations and bugfix

* Add testcases for representation for map and sequence of maps
* Refactor so that Sequable and ILookup are Representations
* Refactor representation generation
* Fix malformed html table (missing tr in thead)
* Render clojure using pr-str and *bind-up* true
* Support Sequable and ILookup as application/clojure
Make UTF-8 default charset. Fix tests.

* Representation uses UTF-8 as default charset if none is given
* Adapt tests to use ring-response where a plain map was used before
* Adapt tests to use a plain map where MapRepresentation was used before
* Add Vary-Header in run-handler (fix)