Restrict ST_ConnectedComponents to undirected graphs.

I will write a function ST_StronglyConnectedComponents for directed
graphs.

Fixes #15. Partially fixes #31 (the rest to be done in #40).
Remove superfluous orientation variable.
Make these public to facilitate future merges.
Merge pull request #53 from agouge/fix/#15

Restrict ST_ConnectedComponents to undirected graphs and remove weight dependency.