Fixed columns with falsey values being pruned

This commit fixes a bug where columns that contained falsey values
would be pruned from the box instance.
Merge pull request #16 from iancmyers/falsey-col-vals

Fixed columns with falsey values being pruned
Bump version to 0.2.8
Created afterFetch hook

Created the afterFetch hook, which will act upon each model object
after it is fetched from the database.

In making this change the #build method has become async. The tests
and internal calls have been updated to reflect this change.
Merge branch 'after-fetch' into sqlbox-mysql

* after-fetch:
  Created afterFetch hook
  Bump version to 0.2.8
  Fixed columns with falsey values being pruned

Conflicts:
	lib/model_methods.js