Add step value for slider, refactor display format for upper and lower thumbs

• Handle stepped values. The slider will snap to discrete points (after dragging stopped) along the slider based on the stepped value.
• Separate display format for upper and lower thumbs

Fix thumb labels FOLLOW alignment

• Thumb labels will align with the thumb correctly in center

Add Carthage support

0.1.1

update docs

First version

0.1

update readme