Separate generic bft-code, rename packages

This splits up the package hierachy into five big sections:

* bft: generic bft engine (will be included in each server)
* trustmanager: responsible for managing SSL certificates,
                might be integrated with ServerConfiguration
                in the future
* frontend: all client-facing functionality (ie. ftp server)
* middle-ware: spliting up commands from the frontend, encrypting
               them and forwarding them to the bft servers.
* backend: backend storage (bft server + storage options)

In addition this adds the library dependency upon the archistar-secret-sharing
library, but does not use it (except for an unit test case which
tests that the integration is working)