chaining test
added mid_x and mid_y methods
converted position methods (#{top,center,bottom}_{left,center,right}) to use same absolute/relative argument as #center method
tests for previous commit
tests for {min,mid,max}_{x,y} and test that CGRect#{x,y,width,height} always return normalized numbers
some documentation regarding absolute vs relative positioning
supports size as an array (and continues to support a CGSize object)
Merge pull request #6 from colinta/master

CGSize * scale and mid_{xy}