Accurate docstring for gm_timestamp

Fixes pycassa#202

pycassa/columnfamily.py

@@ -61,7 +61,7 @@ def keys(self):
         return map(self.name_unpacker, self.type_map.keys())
 
 def gm_timestamp():
-    """ Gets the current GMT timestamp in microseconds. """
+    """ Returns the number of microseconds since the Unix Epoch. """
     return int(time.time() * 1e6)
 
 class ColumnFamily(object):