allow dates back to the 1500s (#230)

As reported in #229 the
oldest natural history records actually date back to the 1500s rather
than 1700.

I have confirmed there are some digitized records in GBIF from the
1600s.

Moving our zero date back a few hundred years.

idb/helpers/conversions.py

@@ -223,7 +223,7 @@
 
 
 def checkBounds(x):
-    lowerBound = datetime.datetime(1700, 1, 2, tzinfo=pytz.utc)
+    lowerBound = datetime.datetime(1500, 1, 2, tzinfo=pytz.utc)
     upperBound = datetime.datetime.now(pytz.utc)
     if isinstance(x, datetime.datetime):
         return x < lowerBound or x > upperBound