Create a constant for 100000000 and use it

Use same name as is in bitcoind.
Note that one of the constants in text.py had the wrong number
of zeroes previously...

lib/util.py

@@ -254,7 +254,7 @@ def parse_URI(uri):
             k = int(m.group(2)) - 8
             amount = Decimal(m.group(1)) * pow(  Decimal(10) , k)
         else:
-            amount = Decimal(am) * 100000000
+            amount = Decimal(am) * COIN
     if 'message' in pq:
         message = pq['message'][0].decode('utf8')
     if 'label' in pq: