test: Remove dead tests, using spec/ instead now
Journal: Start of implementation
Journal: Introduce an initial (null) entry
Journal: Add API for moving between revisions
Graph: Add start/endTransaction API

Will allow journal to group changes more sanely.
Journal: Implement transactions

This gives more meaningful revision numbers:
All initial state belong to revision 0,
all changes from a single API call like .removeNode (incl. iip/edge removal),
and API consumers can manually create transactions and attach metadata there.
Journal: Add undo/redo API
Journal: Use mocha for tests, move to spec/
Journal: move backwards when undoing, apply inverse changes

Avoids invasive side-effects of clearing the entire graph when moving forwards.
Journal: Add to component.json
Merge pull request #139 from jonnor/graph-journal

Initial graph journal