Add options to Layout

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!