Add ST_Accessibility

This adds 
```mysql
ST_Accessibility(output.edges, destination_table | 'dest1, dest2, ...'[, 'weights'][, 'orientation']);
```
where `'orientation'` can be

* 'directed - edge_orientation_column'
* 'reversed - edge_orientation_column' OR
* 'undirected - edge_orientation_column'

The user provides a list of destinations. The function calculates the distance from every node in the graph to each of the possible destinations and chooses the closest destination. This is particularly useful for generating isochrone accessibility maps.

Fixes #26.