test(id): Adding test for id getter

Changing the id property from a function to a getter. Also testing persistence
on multiple instantiations.

tests/specs/id.js

@@ -2,8 +2,11 @@ var test = require('tape')
 var Hoodie = require('../../index')
 
 test('has "id" method', function (t) {
-  t.plan(1)
+  var firstHoodie = new Hoodie()
+  var secondHoodie = new Hoodie()
+  t.is(typeof firstHoodie.id, 'string', 'has id getter property')
+  t.is(typeof secondHoodie.id, 'string', 'has id getter property')
+  t.is(firstHoodie.id, secondHoodie.id, 'id persists upon multiple instantiations')
 
-  var hoodie = new Hoodie()
-  t.is(typeof hoodie.id, 'function', 'has method')
+  t.end()
 })