Fix for ticket #904 - Addition of get_claimed_ip() to HTTPRequest

[moijes12]

As per ticket description of ticket #904, a method to return the first
public IP address from X-Forwarded-For should be implemented. This
update contains the below changes:

1. In tornado/httpserver.py
   HTTPRequest.get_claimed_ip() is implemented which simply returns
   remote_ip
2. In tornado/test/httpserver_test.py
   The test for verifying ip_headers already existed in XHeaders. I simply
   modified the internal class Handler to retrieve the output of
   get_claimed_ip from the request messge and write it to the dictionary.
   The same tests that were written for verifying ip_headers have been
   re-used except that they now check that remote ip equals claimed_ip

[bdarnell]

get_claimed_ip should not simply return request.remote_ip. When a multi-valued X-Forwarded-For header is present, remote_ip should be the last value in the header, and get_claimed_ip should be the first non-private address (i.e. excluding the rfc 1918 blocks and 127/8), with analogous logic for the other xheaders (but simpler since X-Forwarded-For is the only one that takes multiple values). We may even want to interpret X-Forwarded-For for purposes of get_claimed_ip even when xheaders is disabled.

[bdarnell]

Closing due to inactivity.