use functools.wraps to set docstrings on proxies

If you use a tool like sphinxcontrib.autohttp.flask to automatically
document your APIs, it's important that the docstrings on the actual
view functions be extractable.  By wrapping the proxy function created
in make_proxy_method() with functools.wraps(), it is set on the proxy
function and can be extracted by walking app.view_functions
Merge pull request #14 from joeshaw/master

use functools.wraps to set docstrings on proxies