supports custom types

@xaviershay @smparkes 

an attempt at supporting converting from custom data types in postgres to whatever type you define. see individual commits.

interested in hearing opinions on this as an implementation, so i'll just leave these spiders here for you to comment on:
![](http://24.media.tumblr.com/600366aa610efba37bb7e64cd710b9a4/tumblr_mg3jdldeMt1s1llw3o1_500.jpg)