Update response.py

Just something I'm monkey patching locally to allow setting the same header multiple times.

Useful for things like link headers, and setting cookies.
Update response.py
Update test_headers.py
Update test_headers.py
Update response.py
Merge pull request #273 from xiaclo/patch-1

Allow duplicate headers to be appended
test(Response): Improve resp.append_header tests

This patch improves test coverage of resp.append_header and makes the
approach more consistent with the other functional tests in the same
module.
Minor refactoring to allow for restish-like child traversal for
responder location.
Add a property to directly access all parameters.
Handle the case where POSTed keys appears more than once.
Add missing tests - full suite now passes with py27,py34.
Response to code review:

 * Document the params property as a dictionary mapping query string keys to
   values.  Where the keys appear multiple times, the value is documented as a
   list of values.

* Moved the definition of the @property params to colocate with the other
  class properties.

* Added a NOTE(warsaw) as requested.
Merge branch 'refactor' of github.com:warsaw/falcon into refactor

Conflicts:
	falcon/util/uri.py