Version 4.5.0

Enhancements

• PR #138: Optional constraints-based layout.
• PR #140: Add a base class for drag and drop tools, example and test support.
• PR #150: Add a very basic Image component
• PR #160: Basic testing for kiva backends.
• PR #168: Simple push-button and checkbox tool.
• PR #167: Add tools that support Apptools Undo/Redo.

Fixes

• PR#139 Significant code clean-up, modernization, test improvement and
  documentation for Kiva.
• Multiple fixes for breaks to Travis-CI builds.