[INTERNAL] modified SarosContext to allow multi platform support

The SarosContext will no longer create the context but delegate its work
to a context factory.

TODO: the SarosSession also needs a context factory.

Change-Id: I8f0e838bcc29661901137177a428aa274d6d4a5c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1228
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>