Introduced 'whenever' helper for automating crontab updates

Hey guys,  figure you might find this helper handy.
Makes use of the [whenever](https://github.com/javan/whenever) gem. (needs to be on local, and server)

Be careful of conflicts of gemfile / lockfile when merging - I haven't updated my master branch for a while.

With a `config/schedule.rb` file in your project root:
```ruby
every "5 0 * * *" do
	command "/home/www/sites/website.co.nz/current/framework/sake dev/tasks/GeneratedImageCacheControlTask"
end
every "0 * * * *" do
	command "/home/www/sites/website.co.nz/current/framework/sake dev/tasks/CalculateProductPopularity"
end
every "2 * * * *" do
	command "/home/www/sites/website.co.nz/current/framework/sake dev/tasks/CalculateSubShopPopularity"
end
```

Crontab will look like this:
```

# Begin Whenever generated tasks for: website.co.nz
5 0 * * * /bin/bash -l -c '/home/www/sites/website.co.nz/current/framework/sake dev/tasks/GeneratedImageCacheControlTask'

0 * * * * /bin/bash -l -c '/home/www/sites/website.co.nz/current/framework/sake dev/tasks/CalculateProductPopularity'

2 * * * * /bin/bash -l -c '/home/www/sites/website.co.nz/current/framework/sake dev/tasks/CalculateSubShopPopularity'

# End Whenever generated tasks for: website.co.nz


```

Any changes you make, and re-deploy, will update the appropriate section of crontab.