allow unsetting of an attribute
updated Readme
fix toJSON method to allow attributes to be undefined
test for toJSON fix
properly handle null values in toJSON
provide `model.unset`

explicitly states which attributes have been unset, rather than relying
on it being explicitly set to `undefinned`
fix `model.unset` and `model.unset()` naming conflict

changed to `model.unsetAttrs`
use `model.unsetAttrs` in static attr generation
Merge pull request #41 from paulbdavis/unset

Allow attributes to be "unset"