Beginnings of simplifying install of crontab

This is a work in progress. I'm opening this early to get some feedback.

This is addressing #631.

As a first pass I've opted to go down the route of using the current crontab.ugly file and create a rake task for doing the variable substitution (see #716).

The /data/vhost/{name} paths that are sprinkled through the crontab are those used by mySociety. So opted to generalise things a tad and instead we pass the full install directory of Alaveteli. Also, we now store logs and lock files inside the rails directory (in the logs and tmp directory respectively).

Also, went one step further and updated things so that everything can run as the deploy user. It obviously depends on the deploy user having some extra permissions which are now documented in the outputted crontab file.

I really would like some feedback if there are any barriers to this setup being currently used for WDTK.

Then ideally I really would like to go down the path of #610 which will make the cron more readable and maintainable. Is outputting for install in /etc/cron.d rather than using crontab a prerequisite for it being used for WDTK?