clear_cron: skip the -u option if user is empty

[gerhardqux]

Red Hat based distributions use cronie (a fork of vixie-cron / isc cron) that do not allow the use of -u for non-root users, even if it matches the current user.

This results in the error: "must be privileged to use -u"

Call "crontab -r" without "-u user" when the user parameter is empty.

Resolves #48

[jwijffels]

thanks, looks good, can you add a note in https://github.com/bnosac/cronR/blob/master/inst/NEWS, and increase version at https://github.com/bnosac/cronR/blob/master/DESCRIPTION

[jwijffels]

thanks!

[gerhardqux]

thanks!