Add option to update apt cache at compiletime

This pull-request adds a `compile_time_update` attribute to the Apt cookbook to force an `apt-get update` at compile time in order to ensure cookbooks that have an option to run at compile time can install packages. I used `compile_time_update` because `compiletime` is already being used for a different functionality. However, in the future, it may be useful to refactor these into attributes that are more obvious.

##### History
Changes in the build-essential cookbook (discussed [here](https://github.com/opscode-cookbooks/build-essential/pull/32)) have removed build-essential's behavior of running `apt-get update` before attempting to install packages when invoked at compile time. Per the linked discussion in the build-essential cookbook, @sethvargo recommended updating the Apt cookbook to provide an option to run at compile time instead of build-essential being responsible for it.

Full disclosure - these changes are mostly a shameless rip-off of the current compile time implementation in build-essential. :smiley: