Fix: Stars rendering over text

Stars were rendering over text; setting first-level .back elements to have a z-index of -1 (they're already relative positioned) puts them behind the rest of the phase's content.  Since that would also put them behind the phase's background itself, moved the background to a fill sub-element.

• Added a new `.back.fill` div as the first element in each phase.
• Added CSS to `.phase > .back` to have z-index -1.
• Moved each `#…-phase`'s background-color and gradient into a `> .fill` sub-selector.
Minor: Skrollr instance in $s

• Assigned the skrollr instance var to `$s` instead of the recommended `s`.
	» A global var with just one letter is weird.  `$` as an identifier prefix can be taken to mean “static”, so it fits to just prefix the “s”.
	» If I were to keep using this, I should probably map jQuery to `$j` instead of just `$`.  Using more `_` prefixes for local scopes wouldn't hurt either.  Less buzzwords, more consistent conventions.
Minor: All background-position easings to linear

• Changed each of the starfield background-position easing values (back) to `linear`.
	» `swing` didn't really work out like I wanted.  An ease-in (slow start, fast finish) curve might've worked better, but we need it to look good with linear first and foremost.