Added support for kGravatarDefaultImageType (404, mm, identicon, monster...

Added a new helper method to convert from enum to string: ```NSStringFromGravatarImageType(kGravatarDefaultImageType type)```

This will add default image support to your category like this:

```[NSURL URLWithGravatarEmail:userEmail size:40 defaultImageType:kGravatarDefaultImageTypeMonsterId]```