Add enterEdge to Bfs and Dfs.

This function notifies the client about the fact that an edge is
reached for the first time.
Export AdjacencyListGraph and AdjacencyMatrixGraph.

These exports are necessary to allow proper type signatures for functions
relying on this library.
Merge pull request #2 from scolobb/export-graph-types

Export AdjacencyListGraph and AdjacencyMatrixGraph.
Merge pull request #1 from scolobb/add-enterEdge

Add enterEdge to Bfs and Dfs.