Add options to Layout

[siuying]

This patch allow user to set options in NSLayoutConstraint#constraintsWithVisualFormat:options:metrics:views: with simple symbol (like :left, :right, :trailing... etc), and by default use option 0 (same as that in NSLayoutConstraint#constraintsWithVisualFormat:options:metrics:views:).

This patch is necessary because in many use case, the current option NSLayoutFormatAlignAllCenterX and NSLayoutFormatAlignAllCenterY will cause conflicts.

This is based on @wejn patch, added document and examples, and merge with head. Please consider merging this, thank you!

[qrush]

Hey there, I'm looking for a new maintainer for this. Can you check out #13?

[siuying]

no longer using rubymotion in production...