Emit error when APNs notification is larger than 256 bytes

[nhooey]

Currently the pyapns.server.encode_notifications() function doesn't check if its encoded payload size is above the 256 bytes limited by the Apple APNs server.

The function should at least emit an error, but providing a facility to truncate one of the keys' values in the aps dictionary passed to the function down to a manageable size would be best.