Defaulted the Mississipi page selects as requested.

Added code to toggle the sites layer on and off via the Toggle Sites
button. Changed the way the flow lines are added to the map. These are
now added at page rendering and is done before the sites layers are
added. Using setVisibility on layers to turn them on/off.
Modified the maxExtent and restrictedExtent to zoom in to the
Mississippi basin.
Set maxResolution so that the map can't be zoomed out past the initial
extent.
Added download.js to the list of files excluded from testing. This
prevents a console error from being logged when tests are running.
Merge pull request #266 from mbucknell/NAWQAAN-330

Nawqaan 330