Use NSHashTable instead of NSOrderedSet to store checkBoxes weakly

Merge pull request #46 from Boris-Em/feature/offColor

Add new offFillColor property.

Merge pull request #29 from andrey-krukovskiy/CarthageSupport

Carthage support

Merge pull request #19 from xinsight/increase-touch-area

increase touch area if checkbox is smaller than 44pt

Merge pull request #12 from WangHengHeng/master

Fix #13,  Square checkbox of size less than 20 x 20 px appears distorted.

Fixes bug #2 - Check box will be hidden If on property is true by def…

…ault. #2

Added delegate section

Added image