KTp Logger is an abstract interface, losely based on TelepathyLoggerQt
API with support for version backends - currently we only have a
TelepathyLogger backend.

By using this API instead of TelepathyLoggerQt directly we can simply
disable the backend on platforms where TpLoggerQt does not compile
(like Windows) without having to have many #ifdefs all around our
codebase.

In future we can have plugins that would provide access to log messages
from Facebook or Google Hangouts (if they ever publish an API) etc.

REVIEW: 112005