Remove dynamic building of Negotiator class

This is not necessary and in the end makes it slower due to indirect
property lookups.

fixes #32
closes #33
tests: test through the public API
tests: test the singular methods
perf: reduce loop count in header parsing
perf: reduce loop count when calculating priority
Fix list return order when large accepted list
perf: don't alter argument variable
Fix missing identity encoding when q=0 exists
perf: reduce closures

closes #23