#319 Split out Logdoc to a separate project

[Ernst de Haan]

The Logdoc contract-first approach to logging is not specific to XINS and is very useful for other projects as well. So I propose splitting out Logdoc to a separate project and using that from XINS.

A proposed implementation is mostly done, although backwards compatibility needs to be tested more thoroughly. See: http://github.com/znerd/logdoc