Now supports different AlertTypes for major, minor and patch updates.

I created support for different AlertTypes for different updates. These are the default options, and can be changed using the following.
    `[[Harpy sharedInstance] setMajorUpdateAlertType:HarpyAlertTypeOption];`
   `[[Harpy sharedInstance] setMinorUpdateAlertType:HarpyAlertTypeForce];`
    `[[Harpy sharedInstance] setPatchUpdateAlertType:HarpyAlertTypeOption];`

(e.g. Major=2.0, Minor=2.1, Patch=2.1.1)