move Endianness class to overflowconfig module
Changes to overflowconfig.py

* constants for section and key names
* fix splitting of bad character list when parsing bad_characters= value
* add support for other payloads when parsing encoded payload section
Support XOR key of 0x0

* explicitly check if key == None rather than if not key
Changes to Trampoline payload class
* pass endianness,offset to constructor in the right order from reconstitute()
* add pretty_string() method
Fix misspelling of "connectback_ip" when passed to constructor from reconstitute()