Merge pull request #80 from indragiek/master

++RAC
Fix -[RACSignal(RCLAnimationAdditions) animateWithDuration:curve:] implementation
Merge pull request #81 from a2/master

Fix -[RACSignal(RCLAnimationAdditions) animateWithDuration:curve:] implementation
++RAC
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:
Fix merge conflicts
Rename inset:nullRect: -> insetBy:nullRect:
Refactor other inset operators to use -insetBy:nullRect: