Implement automatic night mode

This implements the frequently-requested "automatic night mode", where night mode is automatically enabled after a certain time and disabled after another. Fixes #384.

Here is a screenshot of the settings added to styleTweaks:
![Screenshot of automatic night mode settings](http://i.imgur.com/76kuctL.png)

As noted in the description for the setting, automatic night mode begins at `nightModeStart` and ends at `nightModeEnd`. Automatic night mode can be temporarily disabled (for `nightModeOverrideHours` hours) by clicking on the night mode toggle located in the Gears dropdown menu.

This setting is currently defaulted to "off".