Remove isRequired for iconCategory in Button.

[donnieberg]

Buttons without icons should not be required to pass in this prop :)

[iowillhoit]

Looks good. It would be nice if there was a way to do conditional requires. "If iconName is set, require iconCategory". I'm a little nervous that people will supply an iconName but not realize that they might need to change the iconCategory from the default (utility) to something else. I suppose we could throw a warning..