Implement more eval()-functions

JOSM implements many eval() functions, that pgmapcss currently does not support:
http://josm.openstreetmap.de/wiki/Help/Styles/MapCSSImplementation#Evalexpressions

- [ ] asin, atan, atan2, ceil, cos, cosh, exp, floor, log, max, random, signum, sin, sinh, tan, tanh
- [ ] is_prop_set, has_tag_key
- [ ] regexp_test, regexp_match
- [ ] substring (currently: substr), replace
- [ ] URL_encode, XML_encode, CRC32_checksum