Override 'wdgi.url_scheme' via a request header, 'X_WSGI_URL_SCHEME'.

Allows proxies which serve mixed HTTP / HTTPS requests to control signal
which are served as HTTPS.
Speling
Enforce that 'X_WSGI_URL_SCHEME' must be one of 'http' or 'https'.
Switch to using the quasi-standard 'X_FOWRARDED_PROTO' header.
Typo.
Merge from master.
Normalize long kwargs (one per line).
Add 'trusted_proxy' adjustment.
Only allow trustted proxies to overried request.urlscheme.
Document the 'trusted_proxy' bit.
Merge pull request #42 from Pylons/feature.x_wsgi_url_scheme-header

Override 'wsgi.url_scheme' via a request header, 'X_WSGI_URL_SCHEME'.