set_clipboard_once, allows the selection to be retrieved only once
set_clipboard_once failure handling
python 2.6 fix
better set_clipboard_once and tests for it
set_clipboard_once now works under xvfb
python 2.6 compat -- CalledProcessError arguments
Catch stderr in set_clipboard_once for proper exception reporting

Also display the error from xsel to the user
python2.6 compat
python3 compat
Move clipboard functions and tests into their own modules
use compat module
Merge branch 'clipboard'

Conflicts:
	sdb/passwords.py

Fixes #7