Description of problem:
If the user already has entries in his/her crontab that contains any of the time 'nicknames' e.g/:

string         meaning
------         -------
@reboot        Run once, at startup.
@yearly        Run once a year, "0 0 1 1 *".
@annually      (same as @yearly)
@monthly       Run once a month, "0 0 1 * *".
@weekly        Run once a week, "0 0 * * 0".
@daily         Run once a day, "0 0 * * *".
@midnight      (same as @daily)
@hourly        Run once an hour, "0 * * * *".

The fwbackups will stack trace as such:
Traceback (most recent call last):
  File "/usr/share/fwbackups/fwbackups-runapp.pyw", line 659, in main_close_traywrapper
    return self.main_close()
  File "/usr/share/fwbackups/fwbackups-runapp.pyw", line 697, in main_close
    self.regenerateCrontab()
  File "/usr/share/fwbackups/fwbackups-runapp.pyw", line 609, in regenerateCrontab
    fwbackupCronLines = cron.clean_fwbackups_entries()
  File "/usr/lib/python2.7/site-packages/fwbackups/cron.py", line 248, in clean_fwbackups_entries
    fields = parsedLine.get_all_fields()
  File "/usr/lib/python2.7/site-packages/fwbackups/cron.py", line 99, in get_all_fields
    raise ValueError("Entry is not parsable")
ValueError: Entry is not parsable


Version-Release number of selected component (if applicable):


How reproducible:
Easy was to reproduce is to setup back-in-time and fwbackup to both do backups on FC14.. the defaults will cause errors on them selves.

Backintime's cron tab example:
@daily nice -n 19 /usr/bin/backintime --backup-job >/dev/null 2>&1

fwbackups cron tab example:
21 3 * * * fwbackups-run -l 'davida-lvm-urm' # autogenerated by fwbackups
0 0 * * * fwbackups-run -l 'davida-t-urm' # autogenerated by fwbackups



Steps to Reproduce:
1. Setup backintime daily backup of your home directory.
2. Setup once a month backup with fwbackups.
3. Try to save it (using an backup set).
  
Actual results:
Crontab not updated.

Expected results:
@daily nice -n 19 /usr/bin/backintime --backup-job >/dev/null 2>&1
21 3 * * * fwbackups-run -l 'davida-lvm-urm' # autogenerated by fwbackups
0 0 * * * fwbackups-run -l 'davida-t-urm' # autogenerated by fwbackups


Additional info:
Did not test all eight 'special strings', just the @daily doesn't work.
Don't know when these special nicknames where introduced.  In FC14 look at the extensions sections of 'man 5 crontab' for explanation of 7 of them.