split functions between app.js and arcgis.js

this supports a more clear distinction between functions specific to
this app (app.js) and generic functions for working with the ArcGIS REST
API (portal.js)
updated portal url validation method
changed success popup style to stop pushing header
fixed stats modal hiding
fixed destination login event
changed folder accordion text color
added JSON hint
updated readme
added more url validation
added .gitignore
moved arrayToString to app.js
refactored listItems() to use portal.js functions
removed unnecessary functions