CMake adjustments: removed dependency of qt and wxwidget

ExtendedKey:
* changed key derivation to be done by a separate class: Derivation. Implemented BIP0032 Derivation and Delegation in the BIP0032 namespace
* added support for ExtendedKeys with none matching pub and priv keys - which enables simple split key signing and verification
* updated unit test accordingly

Client:
* Enabled true async operation - async_post/get now accepts a callback that will be invoked upon completion of a call