Adapt YamlImpoter to new constructor of Task and Testcase
Bump psutil version in REQUIREMENTS.txt
Adapt task type Communication to new sandbox

Fixes #126
Communication: Cleanup FIFO dirs after use.
cmstestsuite: Add basic tests for communication tasks.
cmstestsuite: Preserve log files from clean-checkout tests.
cmstestsuite: Add simple tests for contest exporter and importer

Added support for running tests on an existing contest.
Use this to re-create the contest based on an export and run the tests again.

This is clearly not an exhaustive test (e.g. no idea if results are restored),
but does test whether tasks are recreated correctly.

A good next step would be to compare the results from both contests to ensure
they are identical.
Fix OutputOnly.py
cmstestsuite: Actually return non-zero error code if tests fail.
Remove fixed fixme (fixed with new sandbox).
Allow some leeway in wallclock timeout.

If very short timelimits are involved (<= 0.1s), a wallclock limit of 2*t isn't
always sufficient. For example, when communication tasks are involved, the user
executable may be blocked, waiting on the manager to initialise and open its
FIFO. Heavy disk I/O may also briefly stall the execution of a task without
giving it any CPU time.

Give it an extra second for cases like these.