Implement basic working version of toggle layers

 - Every layer must now have an 'id' field, and there are optional
   'legend' fields that allow us to add a layer to the legend.
   Additionally, I made the following changes to the config.yml schema:

  - A 'visible' field indicates whether or not a layer is added by
   default.

  - A 'title' field to provide a layer description on the legend

 - Fixes #4 (toggle layers issue)
Enable MapView to read transparency from the config file
Update config for toggle layers; add NLCD and landuse layers
Fix issue with visibility configuration bug
Update config to not display NLCD by default
Remove extra prints and commented code
Merge pull request #3 from LukeSwart/toggle-layers

Toggle layers