Add support for horizontal scroll views

New:

• fadeAxis
• fadeLeadingEdge
• fadeTrailingEdge
• fadeSize

Deprecated:

• fadeTop (use fadeLeadingEdge)
• fadeBottom (use fadeTrailingEdge)
• fadeHeight (use fadeSize)

Thanks @alloy!

Documentation fixes

• Fix documentation comments
• Add SSFadingScrollViewDemo shared scheme

SSFadingScrollView v1.0

• Initialisers
• IBInspectable properties
• Fade top only or bottom only
• Animated fade in and out
• Adjustable gradient height
• Install via CocoaPods
• Updated for iOS 8+ and Xcode 7