prevent dereferencing undefined & unregister dangling callback

when a frame src changes, and goes from a document with spiritual to
a document without spiritual in scope, there are dangling callbacks
performed on a context that was not spiritualized
Merge pull request #2 from Tradeshift/preventmigrationiframeerror

Prevent error for Migration App iframe
Merge remote-tracking branch 'origin/0.0.4-ts' into 0.0.5-ts

Conflicts:
	pom.xml