Resolve relative symlinks in AvailableTask.
Fix unit tests.

They only used to work because the relative symlinks were resolved using
the current working directory as the point of origin, while it should have
been the symlink's directory.

Commenting the "clean" target in test/etc/tasks/system/AvailableTaskTest.xml
shows that the symlinks created there were actually dangling, but the
resolution algorithm failed to produce the correct path and therefore did not
notice that.
Add tests for absolute symlinks.

Fixes resolution of absolute symlinks and adds tests for it.
Merge pull request #219 from fpoirotte/available_symlinks

Resolve relative symlinks in AvailableTask.
Fixing issue #920: Moving require_once for dependencies to main() call, so that a missing-dependency  won't stop the whole build from working
Merge pull request #222 from synchrone/patch-920

Fixing issue #920: Moving require_once for dependencies to main()
Update Task.php
Fix undefined variable
Fix undefined variable
Merge pull request #224 from tomasfejfar/fix-undefined-var-in-scptask

Fix undefined var in ScpTask
Merge pull request #223 from giorgiosironi/master

Setting location on exceptions only if present