Set to use unittest2 library instead of unittest

This library backports all modern unittest features
to old versions of Python
Added unittest2 so travis can run tests with recent Python
Set unitest2 to be used only on python 2.6
Added support for 'del' in Jenkins object: remove job
Added small delay for test_parametrized_build

so assert will have a chance on fast machines
Added re-try when removing job

Becuase Jenkins sometimes breaks for
unknown reason while removing jobs
Merge pull request #271 from lechat/python3_experimental

Got all tests working on python 2.6 and 2.7