Make output from examples/test more readable

The new logging strategy is to write bokeh server's and ipython notebook's logs to a log file and everything else to a log file and to stdout. Locally you can use `tail -f examples.log` (that's the default file name) to view complete log in real time. On Travis CI the leg file will be written to stdout at the end of test run (no matter of the outcome of the tests). Also, `examples/test` will now complain about output cells in notebooks. By default it will fail an example that contains outputs, but you can also ignore outputs and remove them (the last option modifies the file in-place).