Add nullRect parameter to -insetWidth:height:
Check for CGRectNull after CGRectInset

...instead of comparing manually.
Use CGRectIsNull() to check null rect
Update demos to use nullRect param

Fixes #70 by using CGRectZero as the null rect. Ugly result, but at
least it doesn't crash.
Merge pull request #85 from indragiek/null-rect

Add nullRect parameter to -insetWidth:height: