Many improvements in travis-ci usage

Move travis-ci script into a new file: .travis.build.sh

Since we want to catch test failures, tests should be run during script
and not after_script.  Writing commands in a separate file is simpler.

Enable FreeImage support.

Compile in Release mode, builds run in less than 20 minutes.

Enable OCCT tests.  This requires using xvfb-run on travis-ci, and
tclthread to run OCCT tests in parallel.
Since 'make occt-tests' behave in a strange way, run directly
'cmake -P DrawLaunchTests.cmake'.

OCCT summary output is processed by an XSL stylesheet to print failed
test logs.

travis-ci truncates output when there are more than 10,000 lines of
output.  Builds generate around 9,000 lines of output, trim them to
see test results.
For the same reason, remove OCCT tests which need data files.

Remove tests/xml, these tests hang for unknown reasons on travis-ci.
Merge pull request #471 from tpaviot/review/improve-travis-ci-usage

Many improvements in travis-ci usage