BaseLayerPicker improvements.

1. Add ability for the BaseLayerPicker to have an item specify an array of imagery providers, instead of always using a single provider.
2. Added missing documentation to the ImageryProvider documentation.
3. Cleaned up the BaseLayerPicker code so that it only removes items originally added by the widget, rather than removing the layer at index 0.
Merge pull request #875 from AnalyticalGraphicsInc/baselayerpicker-multiple-providers

BaseLayerPicker improvements.