Automatic width and height

This release contains two smaller new features:

Set alt attribute if none is specified

This change provides a little feature which sets the alt attribute
of the img tag to a reasonable default if none has been specified.

Automatically set width and height if not specified

If no width or height is given, the Gravatar View Helper will now set
the width and height attributes of the img tag automatically, according
to the specified Gravatar size.