Add ability to run a job at yacron startup, like cron @reboot

[evanskinner]

I need to add tests and update the documentation, but before I did that I wanted to get some comments on it.

I'm using yacron to startup a few services and check they are alive inside a docker container, as well as run some actual scheduled tasks.

[coveralls]

Coverage decreased (-0.3%) to 93.452% when pulling 60f4ebcd2f210797b43e7ec62da31b373166cb78 on evanskinner:master into 89013f3f985ec8436e7c4dccd25f9c443e0bf9e1 on gjcarneiro:master.

[gjcarneiro]

Honestly, I don't mind having at @reboot functionality, but would rather have it by allowing a special string "@reboot" to be specified in the schedule field, not as another boolean: it is clearer that way (as long as you document it).

[evanjardineskinner]

Sounds sensible. I'll open up a new pull request for that as I've moved the code to a different fork.

[gjcarneiro]

Superseded by #14.