Current fixes and improvements

Add priority() method to Constraints - now we can wtite something like this:
viewATopConstraint.priority(<~viewBTopConstraint)
and viewATopConstraint priority becomes less than viewBTopConstraint.
Operators: <~ and ~>

Now we can use comparison operators in pinning side notation like this
view1.top(<<=10, to: view2)
and view1 top constraint becomes lessThanOrEqual to view2 top constraint with 10pt offset
Operators: <<= and >>=

Also, now we can write pinnig sides without offsets
view1.top(.bottom, of: view2)

Fix bug with center(x:y)