ExpectedSystemExit improvement in multi-threads environment

[akryvtsun]

trafficstars

Implementation of #44 fix

[akryvtsun]

Implemented your notes. Fixed FindBug warning.

[stefanbirkner]

We are getting closer to a final version of the pull request. Thank you for your patience.

[akryvtsun]

@stefanbirkner you have to supprese Firebugs check for intentional ignoring return value in line exitStatusHolder.offer(status); somehow.

[akryvtsun]

Here is information how to do this http://stackoverflow.com/questions/1829904/is-there-a-way-to-ignore-a-single-findbugs-warning