add action to abandon a change

Resolves #48
improve handling of Git errors
add troubleshooting hint

- Checking out from VCS with Gerrit plugin does not work
add settings in push dialog

- add setting to enable pushing to Gerrit by default
- add settings for reviewer, draft changes, submit changes and topic in push dialog
- extension of dialog is done javassist because of missing entry points

Resolves #47
Merge remote-tracking branch 'origin/master' into intellij13