Improved property downloads and added a graphing function with d3.js

OOSMStationInfoViewController no longer determines which property a callback corresponds to.  The callback includes a string which indicates the property it represents.  OOSMWebViewController was added to the project.  This View Controller displays a chart generated using D3.js to display a historical graph of the property selected for a certain station.  This resolves asascience-open/oos-mobile#3.  A launch image was also added which will immediately show the user content while the map and its pins are loading.