Extensionize SMS - convert clickatell to an extension

Main idea is it to make adding new providers as extensions without any requirement for modifications to core.
Each provider should be able to configure / provide its own handler.

Ideas to experiment while we working on it:
===================================

• Keep SMS as internal extension part of core. Assume a default extention directory that works without any configuration.
• Schema changes could be injected (or removed) dynamically when extension is installed / uninstalled.