Image Selector

change image selector labels to block elements (b150110)

added disabled and error states for image selector (a63c5cd)

properly extend form checkbox, radio, and image selector interfaces for docs (7457647)

move some sass rules to layout (2390806)

fix element selector, radio example (d8efe6a)

apply all CoreImage attrs; use legends for examples; change selected border size (7981f66)

basic implementation for image selector (6800e39)