add `message` back to `Function` and emit warning

fixes #64
Merge pull request #127 from claytron/message_deprecation

add `message` back to `Function` and emit warning
a timezone of Z means UTC

Adaptation of patch presented here:
http://code.google.com/p/pyiso8601/issues/detail?id=15
default_timezone was completely untested
sign contributers
forgot changelog
Merge remote-tracking branch 'upstream/master'

Conflicts:
	CHANGES.txt
	CONTRIBUTORS.txt