Load <identity> element from app.config

[Strachu]

WcfClientProxyGenerator in current version seems to ignore <identity> under <endpoint> tag in app.config. We need to use it along with windows authentication.

This pull request adds loading of <identity> tag during the creation of a proxy. There is no test for a rsa identity as I could not find an example how to specify it. The example from official documentation seems to not work results in an exception even when using it without a library.