Split off crontab read/write to CrontabFileHandler.

This reduces bloat in the Crontab class and aids the separation of
concerns. Backwards compatibility is kept, but the old methods are
marked as deprecated.

Also added functionality to read and write Cron Jobs from arbitrary
files.

Signed-off-by: Jacob Kiers <jacob@jacobkiers.net>
Merge pull request #7 from alphacomm/master

Split off crontab read/write to CrontabFileHandler and add read/write of arbitrary files.