Implement more eval()-functions

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