Add ST_ConnectedComponents

`ST_ConnectedComponents` calculates the connected components (for undirected
graphs) or strongly connected components (for directed graphs) of a graph.  It
produces two tables (nodes and edges) containing a node or edge id and a
connected component id. Signature: 
```mysql
ST_ConnectedComponents('input_edges', 'o[ - eo]')
```

where 
* `input_edges` = Edges table produced by `ST_Graph` from table `input`
* `o` = Global orientation (directed, reversed or undirected)
* `eo` = Edge orientation (1 = directed, -1 = reversed, 0 = undirected).