sendUpdate() can now handle a receiving engine 0, i.e. no engine is specified. In that case, no attempt to compute a known ancestor is made.
added deleteBranch()
Added a setBranchTip method while removing duplicated code that did essentially move a branch
Improved the behavior of addEngine(). I'm pretty sure the engine listing for each state was inconsistent before
Added a getBranches() method
Added getEntities() and getStates() methods