Updates for Carto 0.9.4

Looks like there were a few breaking changes in Carto 0.9.4 that we never caught up with.

1. eval() vs. ev(), as mentioned in #62
2. Changes in the format of `filters` in the AST. Hopefully what I'm doing here is ultimately the right thing, though it might not be. Seems like the Carto AST has perhaps gotten a little more Mapnik specific.

Fixes #62. You may want to try playing with this, @hampelm/@prashtx.