Bug fixes for state sync issues

• Fix to deal with changes to size and anchor properties on anchored spaces and adjusted spaces accordingly.

Extra space properties and state sync issue fixes

• Added 'as' property to allow rendered DOM element to be specified.
• Added 'centerContent' property as short-hand for centering space content instead of having to introduce the extra <Centered /> or <CenteredVertically /> components.
• Allow resize handle width to be specified and also it's placement within space controlled with the 'overlayHandle' property.
• Fixed issue with dynamically added/removed anchored spaces where an anchored space would see itself as a space taking space.

package.json fixes

• Added repository field to package.json

Space CSS styling fixes

• Removed clear fix from space

Fixed resize handles on scrollable spaces

• Fixed positioning of resize handles within scrollable divs

Resize constraint fixes

• Fixed resize minimum and maximum constraints

Bug fix for resize handles

• Fixed resize handles being covered by nested spaces preventing resizing

Implemented percentage sizing on anchored spaces

• Add support for percentage sizing on anchored spaces

Documentation/readme fixes

• Added readme
• Updated documentation

Added trackSize property and VerticallyCentered component

• Removed ResizeSensor from spaces by default and now optionally allow live size updates with trackSize property
• Added <VerticallyCentered /> component to vertically centre content within a space
• Allow class names to be specified on top-level spaces <ViewPort /> and <Fixed />