Improve color handling

[xamanu]

This is the implementation of #104 to improve color handling and move essential parts into the core part of osm2gtfs. This PR includes:

• Line objects' route_color attributes are now a hex value (and shouldn't be different things).
• Data from OSM with colour information is sanitized (checked and eventually converted from W3C color names to hex values).
• The route_text_color can be obtained from OSM by using the ref:colour_tx tag. This seems to be the most appropriate, when looking at the wiki. Of course creators are free to use any other tag.
• If no information on the text color has been found in OSM; the standard RoutesCreator computes it's own route_text_color, based on a smart logic found by @ialokim and implemented within the helper function calculate_color_of_contrast. Of course creators can override this standard creator behaviour to define the colors and the logic as they want/need.

This PR is based on #99 (data structure).

[ialokim]

Looks very nice, thanks @xamanu !

[xamanu]

Rebased on latest upstream master branch

[grote]

Thanks, looks good to me!

[ialokim]

I've only rebased on current master and squashed the two commits into one. I think this is really ready to be merged!