Move educe.glozz model into educe.annotation

My current thinking is to have two abstract layers:
- annotation
- graph

We only have the annotation layer for now.  The way I current see things
evolving in the future

Abstract layer:

-    **educe.annotation**: low-level, close to annotations
-    **educe.graph**:  high level, graph-like structure
-    **educe.corpus**: abstraction over corpus organisation

Format/formalism layer:

-    **educe.glozz** : read Glozz XML files into educe.annotation
-    **educe.sdrt** : convert educe.annotation to educe.graph
     (cf a possible educe.rst, which does same)

Data-set layer:

*    **educe.stac**: things which are specific to a particular
     data set; encodes filename conventions and various
     task-specific/ad-hoc choices made along the way
     (cf a possible educe.annodis...)