Map HTTPS server variable to X-Forwarded-Proto header

In express.js, "trust proxy" mode allows is to properly handle the
scenario of being behind a proxy server, while still properly exposing
the client IP/protocol from the request object.

The enableXFF configuration option for iisnode propagates the
X-Forwarded-For header, but not X-Forwarded-Proto. This commit adds the
X-Forwarded-Proto header as well, in scenarios where enableXFF is true.

For details, see:
https://github.com/tjanczuk/iisnode/issues/265
Fix typo in README

Build instructions were missing a closing '%' on a variable name.
X-Forwarded-For shouldn't include port

The WSAAddressToString function used to convert the client IP to a
string was also including the client port in the result. According to
the "spec" for X-Forwarded-For, it is only supposed to include the
numeric IP address.

I've switched the code to use the GetNameInfo function instead, with the
NI_NUMERICHOST parameter. This properly returns just a formatted IP
address, and works for both IPv4 and IPv6 addresses.
Clean up X-Forwarded-* implementation

1) define statics in header
2) Convert strcmpi to _stricmp
3) Only add X-Forwarded-Proto if it doesn't already exist in the
incoming header (+ add test case for this scenario, too)
Merge pull request #310 from dpolivy/xff

Support for X-Forwarded-For and X-Forwarded-Proto headers