Replace mapnik with simple tiles

[eliotjordan]

Replaces mapnik library with the (simpler) simple tiles library for generation of vector thumbnails. Returns almost identical images. Also creates a new Image processor module to hold all the Imagemagick methods.

[drh-stanford]

do you have a screenshot of what the new thumbnails look like?

[eliotjordan]

@drh-stanford screenshots. I tweaked the default colors, but they're configurable.