fix(Request.client_accepts): Accept parsing doesn't handle application/*

This patch adds support to the parser for robust accept header handling,
courtesy of mimeparse. Care was taken to make the common cases fast.

As part of this change, Request gains a new client_prefers() method that
can be used for content negotiation when the service supports multiple media
types.

Fixes #155
Merge pull request #176 from kgriffs/issues/155

fix(Request.client_accepts): Accept parsing doesn't handle application/*