Added Penner easing equations, simplified

• Added Penner easing equations.
	» Derived from https://github.com/danro/jquery-easing/blob/master/jquery.easing.js)
	» Simplified down assuming a the beginning-value of `0`, change-in-value of `1`, and duration of `1`.
	» Also added whitespace around all operators, moved in-equation mutation (e.g. `(t -=1) * t * t + 1`) to before the equation, turned some divisions into faster multiplications, added proper indentation after `if`s, and turned an odd ternary into an `if`.
	• Added each to skrollr's `easing` options without its `ease` prefix.
		» This overrides skrollr's default `outCubic`, which is probably for the best since it uses `Math.pow` when a multiply would be faster (IIRC).
Added `…MidToEnd`/`…StartToMid` variants for Penner easings

• Added `…MidToEnd`/`…StartToMid` variants for each of the Penner easing equations supplied to Skroll's options.
	» In a new 1-line format using `lerpStartToMid()`/`lerpMidToEnd()` to transform the `p` value prior to passing it the `ease…()` function.
• `linearMidToEnd` and `linearStartToMid` became stand-alone `lerpStartToMid()` and `lerpMidToEnd()` functions.
	» Passed directly to `linearStartToMid` and `linearMidToEnd`.
	» Used in each penner `*StartToMid`/`*MidToEnd` easing option (see above).
Quarter easing functions / alpha tweaks

• New `lerpStartTo3rdQuarter()`, `lerp1stQuarterToEnd()`, and `lerp1stQuarterTo3rdQuarter()` functions map the linear range to ⅓ or ½ the range, capping to 0 or 1 at the moved range edges.
	• New `*StartTo3rdQuarter`, `*1stQuarterToEnd`, and `*1stQuarterTo3rdQuarter` Skrollr easing option variations re-map the input's range, then pass it to the named easing function.
• Changed starfield2's `@opacityEasing` to `outQuart1stQuarterTo3rdQuarter`, starfield3's to `outExpoMidToEnd`, and starfield4's to `outQuadStartToMid`.
	» To help make the star appearances spaced out but noticable.
• Changed starfield4's `@initialOpacity` from `0.25` to `0.0`.
	» Cleaner look with only 1 field initially shown.
Tweak: Starfield scrollOffsetFactors

• Adjusted starfield4's `@scrollOffsetFactor` from `0.25` to 0.2`.
	» To recede into the background better, more close to starfield1's `0.15`. 
• Adjusted starfield2's `@scrollOffsetFactor` from `0.333` to 0.3`.
	» To stand out a little better from starfield3's `0.5`.
Fix: New .back.clip wrapper

• New `.back.clip` wrapper div for sky-phase's `.back.fixed` element.
• New back `&.clip` CSS `position`s itself as `absolute` and applies an `clip()` at the edges of itself (`auto` for each side).
	» Selector evaluates to `#sky-phase > .back.clip, #sky-phase > .back .back.clip`.
	» The `absolute` position is `!important` to ensure other CSS hasn't overridden it— `clip()` doesn't work unless it's on an element with absolute or fixed positioning.
	» Clipping the content like this helps iOS know when to redraw the next phase's content because there's nothing present (other than the body) where the ambitions-phase needs to be.  It also seems to help performance as whole— the browser knows that it only needs to redraw the screen area, not the whole phase's height.
Fix: Nesting `& when not (…) and (…) {`

• Nesting the two conditions of `.generateStarfieldRules()`' `& when not (…) and (…) {` separately to achieve the same intended effect.
	» LESS's implementation appears to be buggy— when passed an @initialOpacity of `0.0` and an @endOpacity of `0.5` it incorrectly compiles out the contents.