Start wait_callback after setting self.pipe, to avoid crash in _wait_for_end when the process takes a while to start
Adds unit test base.
Add ExternalProcess tests.
Fixes AsyncPopen.pipe not defined (Closes ArchiveTeam/seesaw-kit/#26).

Merge commit '209480' into development

Conflicts:
	seesaw/externalprocess.py
Fixes off-by-one (Closes ArchiveTeam/seesaw-kit#29).
Reduce unittest waiting time for ExternalProcess.
Unittest assertIn fix for 2.6