Fix for silent plugin failures.
Better error message with less code. Stacktrace only logged in verbose.
Updated error message when junitxml report cannot be created
tests._common now captures output even if SystemExit is caught.
Moved error handling from events to main.

This makes the plugin code match the unitest2 plugins branch more
closely.
Also, sys.exit is safer to call from the main module.
Prototype for Layers plugin detection in Such
Only submit coveralls build for python 3.3
Added cov33 which gives 80% coverage compared to 53% for cov27
Omit backport coverage
Merge branch 'fix_issue_159' into ab_master
Merge branch 'fix_issue_201' into ab_master
Merge branch 'fix_coveralls' into ab_master