Document PYTHONIOENCODING for UTF-8 IO handling

[jbyers]

On Ubuntu 14.04 / python 2.7.6, we encountered the error below when a command emitted UTF-8. The solution was to signal to python that stdin/stdout/stderr were UTF-8 encoded by crontab environment variable:

PYTHONIOENCODING=utf8

This patch adds this as a note to the README.

Traceback (most recent call last):
  File "/usr/local/bin/cron-sentry", line 9, in <module>
    load_entry_point('cron-sentry==0.7.0', 'console_scripts', 'cron-sentry')()
  File "/usr/local/lib/python2.7/dist-packages/cron_sentry/runner.py", line 122, in run
    sys.exit(runner.run())
  File "/usr/local/lib/python2.7/dist-packages/cron_sentry/runner.py", line 159, in run
    sys.stdout.write(last_lines_stdout)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 175-268: ordinal not in range(128)

[hltbra]

Thanks for the documentation contribution, @jbyers!

[jbyers]

No problem - thanks for the fast merge, and for the most useful utility.