Use request.path_info when determining the selected page.

Fixes gma/nesta#112.

This just implements the fix noted by @ms. Using request.path_info is the correct method because it ignores the SCRIPT_NAME environment variable that gets set when a Sinatra app is mapped to a path other than "/". 

I think other than proving it doesn't break existing functionality, adding a test to show that SCRIPT_NAME is in fact ignored is all that is really needed.