Added ESRI JSON sample source to tests
Fixed missing file extension bug for Alameda County data
Minor touch-up and removed various mentions of urllib2 since it's now requests
Using hashed URLs for cached data in download tasks
Found it necessary to use matching modes on pickle file

When debugging on Ubuntu 14.04, discovered that mixing 'r+' and 'r'
was generating bad file descriptors and causing exceptions at lockf().
Switched to faster cPickle

Found that non-fast pickle was throwing value errors.
Added notes about unpredictable process.join() bugs on Mac