width/height methods taking percentage parameter

Add width/height methods taking percentage parameter
* width(_ percent: FPercent)
* height(_ percent: FPercent)
* minWidth(_ percent: FPercent)
* maxWidth(_ percent: FPercent)
* minHeight(_ percent: FPercent)
* maxHeight(_ percent: FPercent)

Usage examples:

* view.flex.width(50%)
* view.flex.height(25%)

* Added by [Luc Dion](https://github.com/lucdion) in Pull Request [#28](https://github.com/lucdion/FlexLayout/pull/28) 

Add Carthage support

Add Carthage support

• Added by Luc Dion in Pull Request #25

Improved the Align enumeration

Replace the unique Align enumeration by 3 enumerations:

• AlignContent
• AlignItems
• AlignSelf

They all have different options, which make the API cleaner.

Plus:

• Removed the overflow() method for now.

addContainer() renamed addItem(), markDirty()

• Add missing markDirty() method
• ⚠️ BREAKING CHANGE: Renamed the method addContainer() to `addItem(). It is clearer that the added view is in fact a flex item, and not just a flex container.
  • Added by Luc Dion in Pull Request #8
• Add an implementation of the Ray Wenderlich Yoga Tutorial
  • Added by Luc Dion in Pull Request #7
• Add Swift 4.0 support

Initial official release

• Initial official release.
• Add unit tests

Initial beta release

0.1.1

Update doc + pod version