Major refactor of predicates and policies.

Now only DynamicPredicate and DynamicPolicy subclasses change.
Previous primitives with changing behavior such as flood, ingress_network
and egress_network are now subclasses of DynamicPredicate or DynamicPolicy.
The only exception being query policies which are their own thing for now.

Previous primitives such as move and modify have been subclassed from
DerivedPolicy as well, and semantics have been normalized (see changes to
examples).

A useful cross-forward DerivedPolicy has been added (semantically equivalent
to OpenFlow's fwd(port_num) action).
basic core language documentation
Added installation/setup instructions and link to video directory
added first pass at of_tutorial-style file