tests: Use xtestenv for travis-ci unit tests.

The C++ unit tests are dependend on a running XtreemFS environment. Running the
tests within the xtestenv ensures it is available.
To be able to distinguish between C++ and Java failures, the xtestenv has been
extended to write the test results to a json file.
tests: Add option to show progress to xtestenv.
tests: Fix openjdk6 test compatibility.
Merge pull request #335 from kleingeist/jdillmann-travis_cpp

Use xtestenv for travis-ci and re-enable c++ tests.