YEAR and SPAN variables

In an attempt to minimize errant changes to the base URL, created YEAR and SPAN variables.  
No changes to base URL are needed unless Census API Call changes.
Valid values for YEAR are greater than or equal 10.  I.e, 2010, 2011, etc..
Valid values for SPAN are 1, 3 and 5.  Note, tract data is only available at the 5 year level.