Enabled test coverage for source code that isn't touched by a test file.

Previously, if a file had zero percent test coverage, Istanbul would
happily ignore it. Istanbul is now configured to instrument source code
that isn't used by any particular test.
Minor cleanup of the Connection class.

Added some documentation and cleaned up the connection class
declaration. The class itself is now properly exported.
Minor cleanup of the Graph class.

Added some documentation and cleaned up the graph class declaration. The
class itself is now properly exported.
Minor cleanup of the Node class.

Added some documentation and cleaned up the node class declaration. The
class itself is now properly exported.