Handle non-ascii headers in multipart/form-data

Second stab at shazow/urllib3#119, now integrated with the RequestField
infrastructure.
Move main logic of _render_part to stand-alone function format_header_param.
Merge branch 'indentation' into issue119b

Conflicts:
	urllib3/fields.py
Simulate a standards-compliant web server.

This is an ugly work-around for facebook/tornado#868: we override one of
their internal functions, hoping that they don't decide to change its
semantics.
Avoid incomplete coverage due to Python 2 compatibility.

Moving the Python 2 code on the same line as the condition should trick the
coverage computation into treating the whole thing as a single statement,
and since the if part will be processed, it should be covered. Nasty.
Less hackish way to exclude Python 2 code from coverage.
Merge pull request #223 from gagern/issue119b

Handle non-ascii headers in multipart/form-data