Begin experimental use of exceptions.

Provides IqTimeout and IqError which are raised when an Iq response
does not arrive in time, or it arrives with type='error'.

sleekxmpp/exceptions.py

@@ -52,3 +52,16 @@ class XMPPError(Exception):
         self.extension = extension
         self.extension_ns = extension_ns
         self.extension_args = extension_args
+
+
+class IqTimeout(Exception):
+
+    """
+    An exception which indicates that an IQ request response has not been
+    received within the alloted time window.
+    """
+
+class IqError(Exception):
+
+    def __init__(self, iq):
+        self.iq = iq