Added possibility to extend proxy headers

The additional parameters were added to expand image proxy settings, for example, passing on the authorisation header to proxy. In particular options accepts the following settings:

• headers - proxy server request headers
• onSuccess - callback will be called after the successful response from the proxy server.

The fixes done in the proxy-with-auth branch made from the v1.0.0-rc.1 mainstream repository tag, that should be used until the main repository fixes all proxy related issues.