Handle plugin frequency values from configuration
- add get_frequency(), map_time_unit(), test_get_frequency(),
  test_map_time_unit()
- use get_frequency() to fetch the plugin frequency from the cfg
- honor environment variable $CRAWL_LOG for setting logfile path
- update README.md with info about setting logfile path
Make the plugin run at fire time
Robustification
- Catch tracebacks from the plugin in CrawlDaemon.fire()
- Wrap a loop around the contents of CrawlDaemon.run() so we don't die on a
  random plugin failure
- In tests, check for 'crawl start' failing because there's still a pid file
  lying around
- Generalize get_frequency() to get_timeval()
- Prefer '-' over '_' where possible (plugin-dir, e-mail-recipients)
Use get_timeval instead of get_frequency()
- Call get_timeval instead of get_frequency()
- Remove function get_frequency()
- Move test_get_timeval() to right place in test order
- Log ConfigParser errors in get_timeval()