Support customized tornado_settings (issue #22)

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I didn't find a regression test suite, so my testing consists of applying the change to our own development environment (along with some temporary debug logging code, not included in the patch), and running it with a modification to our own class derived from SchedulerServer to invoke the base class constructor with a dictionary of our own override for one of the tornado settings passed as the new custom_tornado_settings argument. The patch appears to behave as expected, without breaking anything. If you have any private test suites not included with the repository, please run them.

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Fixes issue #22.