Fix required properties with zero value errors.

'required' properties that have a zero value
generate an error when they should not. This is
due to if (source[property]) check in getProperty,
instead of (source[property] !== undefined)
Merge pull request #52 from bjnortier/required-property-with-zero-value-generates-error

Fix required properties with zero value errors.
Merge branch 'master' of github.com:Baggz/Amanda
Fix ‘TypeError: Object [object Object] has no method 'addError'’