Expose number of vertexes and edges
(cherry picked from commit cb53ee8450d1de3ef8a6d383d5d1baab43228f57)

Conflicts:
	lib/Network.ml
Fixed add_edge to prevent duplicates for same src/dst vertexes and ports
(cherry picked from commit e18f2e1332b6736e6e7af89e2539c4465fd80d83)
Make add_edge assumptions explicit
Merge pull request #34 from frenetic-lang/fixes

add_edge bugfix and add convenience functions