Proxy to external URLs

[emmdurin]

First attempt to add a proxy to external URLs, see issue #39
As of discussed in the issue and in some meetings, it may be a bad idea to add a such proxy to the gateway and it is yet to be decided, but for reference here is a working attempt without any security, compatible with the one at /proxy in security-proxy.

Example of proxy route declaration :

      - id: proxy
        uri: no://op
        predicates:
        - Path=/proxy/**
        filters:
        - Proxy