Implement simple image exporter for the imported XML

- Allow filtering of ways so that only highways are shown (or all ways if the filter_ways() method is not called)
- Add some minor documentation
- Change way how the module is imported
- Update readme to reflect change in requirements
Add filter for nodes

Nodes are removed if they don't satisfy the criteria:
- Node contains some data (has tags within it)
- Node is referenced by a way
Add a command line option to specify a different map, still use the test by default

- First command line argument is the name of the map file
- XMLReader now has ability to load a file from the constructor
Very simplisticly convert XML to a graph

- Use ways as nodes
- Edges between ways are only added when they share a common end point
- Add some requirements