2.0.0.beta.20 released

[rsnyder]

This update was primarily focused on adding new functionality to the Map Viewer . The updates include:

• Add support for alternate base layers
• More flexible definition of overlays in the .ve-map Markdown list, including the use of a layer attribute for assigning overlays to groups
• Added controls for selecting/deselecting base layers and overlays and for adjusting the opacity level of image overlays
• Various bug fixes and optimizations