Simpler request helper functions that accept bytes and unicode args

This is related to a bug I wrote a while back #108 for Klein and 8905 for Twisted. It would make writing code more pleasant if certain functions of the Request object would accept both bytes and unicode. This would also bridge gaps between writing Py2 and Py3 code. This issue was primarily created because there are inconsistencies with:

• Headers (param can be bytes or unicode)
• adding cookies (param can be bytes or unicode)
• getting cookies (param can only be bytes)
• getting request.args (param can only be bytes)

It is easy to do something like 'unicode or py2 str key'.encode('utf-8') before passing it to the various getter/setter functions, however the more popular Python web frameworks never force users to do this and Klein should follow suite. I'm well aware that these are upstream Twisted issues, but it's extremely noticeable in Klein. My hope is to deploy a solution here, test it out with the people who use Klein, then merge them upstream.

Start with Klein's Request object and compare the handling of Headers, addCookie, cookie lookup, and request.args, then review related issues #108 and Twisted ticket 8905. The work is complete when the affected helpers consistently accept both bytes and unicode and the behavior has been tested with Klein users before considering an upstream change.