Add option to pass in default gravatar parameters

I was looking through the default gravatar options and I noticed you can pass in special parameters to the url to get different avatars, like the one StackOverflow uses for example. I added an option to pass in default gravatar parameters via a 'defaultAvatarParams' property to make this easier, since I noticed the Gravatar.imageUrl function pretty much already implemented the logic required. The options I'm talking about are the ones [here](https://secure.gravatar.com/site/implement/images/#default-image). 

Oh, and since the Gravatar package does some string operations, if you happened to pass in an undefined value into Gravatar.imageUrl, an error would be printed to the console. A possible scenario might be if the emailHashPropery is set in config, but the user hash property didn't exist. I added a simple check to make sure that this doesn't happen, and return the defaultUrl if Gravatar.imageUrl fails.

Great package by the way!