Properly parse nested proxy

[whs]

Closes #6

Only continue parsing IP backwards if the proxy is in allowed IP range.

This PR breaks the public interface by introducing allowed function parameter into Parse. Also the Parse behavior is now changed:

• If invalid address are found, the last valid IP are returned instead of silently skipping those invalid address.
• Local IP address can now be returned (IsPublicIP become unused function) as all local proxy IP should be listed in AllowedSubnets.

I believe this breaking change is justified as it prevents people from having security issues by not providing insecure validation mechanism. It should be released as versioned package though (eg. using gopkg.in)

[whs]

Found some more bugs where it wouldn't work with internal proxies. Looking into this