Add CreateChannel overload for duplex services

Fixes #9. Updated functional test to include callbacks with a duplex
service contract.
Always close and abort the channel

Even if dispose isn't called, catch close errors and abort on finalize.
Add example for duplex WCF

Demonstrates both async and synchronous callbacks.
Update changelog for duplex WCF
Merge pull request #14 from spanglerco/duplex

Duplex WCF