Make Field Error Class settable

Fixes #501

This change adds an argument to `NewField` to set the `errorClass` for the field. I have updated all the examples, but this is an API change, and anyone using `field` in their templates will have to add an extra argument. If there is a page for upgrading from the previous version, if you are fine with this change, I will add a note about this.

BTW, I wasn't able to do a straight rebase without nuking my repository since I had already pushed, so I did a merge into my `origin/develop` but this might have picked up a few commits in `master` that weren't in `develop`.