Change unmanaged property to unnamed

This gives a much better sense of what the property measures as named jobs can
also be unmanaged.
Guard the parslet provider with a feature

Add a Puppet feature that checks for the parslet library and use it to confine
the provider.
*** (FIX) *** Handle nil values when formatting

Soooo... not everything defaults to `:absent`. The Type does leave most things
as `nil`. Probably done so that values read in from the file can be used as
dynamic defaults instead of specifying a single value in the type.

This patch kinda sucks. It can be done better.
Monkeypatch core cron provider with custom facts

This is the most heinous, abusive and evil thing I have ever done to Puppet.
But, it allows the parselet provider to take over without interference from
the core cron provider.