Disable Idle Timer During Downloads

This changeset does the following:

* By default, DocSetDownloadManager automatically disables the device's idle timer when the user starts a download and reenables it when all downloads finish. This helps users whose downloads take longer than the time it takes the device to auto-lock + the maximum allowed time for background tasks.

* DownloadViewController displays a toolbar when downloads are in progress. The toolbar contains a switch that allows the user to override the default behavior. The override setting is _not_ persisted to the user defaults.

![docsets-screenshot-idle-timer-toolbar](https://f.cloud.github.com/assets/5010/131907/3162aa3c-7072-11e2-8768-651b55743f00.png)