refactor backup failure logging
fix syntax warning
fix Timecop usage
add yard to Gemfile
remove test files from distributed gem
add log to syslog

This commit adds the default behaviour to log all messages to syslog LOCAL0
facility as well as the regular log file.
fix simple syslog tests
Update Logger to better support Syslog addition

- Adds command line options --syslog/--no-syslog,
  as well as --logfile/--no-logfile to disable logging
  to Backup's native log file.
- Logger may also be configured using Logger.configure {...}
  within the user's config.rb file. This was primarily to allow setting
  other Syslog options. To support this, the `backup perform` startup
  process was modified and the Logger now buffers all messages it
  receives until configuration can be completed.

Other side-effects:

- `backup perform` will now exit with an error if no Models can be found
  for the triggers given. Previously this would not occur if the trigger
  used a wildcard.
- If two Models have the same trigger, they will now both be performed.
  Previously only the first found Model would be performed.
update Notifier::Mail to use new Logger
update spec-live for Logger changes
Logger.message -> Logger.info
Merge branch 'syslog' into develop
backup perform `--check` option
bump Mail to ~>2.5.0 (fixes #336)