[Tilecache] Two simple patches

[Alex Morega]

Hello,

I wrote two simple patches that were useful to me:

https://github.com/alex-morega/tilecache/compare/5fc8896e...1c65e405

One tries to import "mapnik2" if it's available, and falls back to "mapnik"; the other one creates a new cache backend, "Null", that doesn't actually do any caching. Easier than removing files from disk all the time during development.

Is it realistic to have them included? Anything I should do to improve them?

Thanks,
-- Alex