New colors for the sky

• New colors `@sky-eve-top`, `…-mid`, and `…-bottom` for the starting gradient of the sky.
	» `…-top` and `-bottom` are specific RGB values.
	» `…-mid` is the `overlay()` result of the top and bottom, because it thought it looked nice.
	• `@sky-eve-top-mid`/`-bottom-mid` provide averaged (mixed 50/50) quarter-points, in case we need them.
• New colors `@sky-night-top`, `…-mid`, and `…-bottom` for the ending gradient of the sky.
	» Based on `@…-pure` colors, but proportionally darkened via `multiply()` calls, since the “pure” colors were a bit bright.
	• `@sky-night-top-pure`, `…-mid-pure`, and `…-bottom-pure` are colors I like, specified with RGB values.
	• `@sky-night-top-mid`/`-bottom-mid` provide averaged (mixed 50/50) quarter-points, in case we need them.
• Added two new rows to `test/color-swatches.html` for the 5 `sky-eve-…` and 5 `sky-night-…` colors.
Minor: Added subtle borders to color-swatches

• Added 1px borders to the test `.color-swatch`es.
	◦ 20% opaque mid-gray in the general case
	◦ 10% opaque white when encompassed by `.on-dark`
	◦ 10% opaque black when encompassed by `.on-light`
Fix: Added `on-dark`/`-light` classes to color-swatches.html

• Added `on-dark` and `on-light` classes to each of the `col-…` divs in `color-swatches.html`.
	» The ones already with `on-left` got `on-dark` and the ones with `on-right` got `on-light.
	» I thought these were already on here.  Unsure why I removed them.  Hmm.
Fix: `-as-rgb()` colors using non-existent `@as-hsl`

• Changed `._*-as-rgb()` lambdas to use the correct `@as-rgb` var.
	» Many of the existing `._*-as-rgb()` lambdas were accidentally setting their returned `@…-as-rgb` var to `@as-hsl`, which is non-existent in that scope, which I guess defaults on the JS side to null and gets somehow processed into black.  Like weird that we didn't get an error.
Fix: Commented-out gradient prefix code in Skrollr

• Commented-out the portion of code in Skrollr's internal `_parseProp()` function that prefixes gradient rules with the browser's vendor prefix (e.g. `-webkit-`).
	» The `-webkit-linear-gradient` code it's generating is invalid and ignored by the current versions of Safari, iOS's MobileSafari, and Chrome.  I'm assuming the differences between W3C's `linear-gradient` and WebKit's final `-webkit-linear-gradient` rev are enough to throw it off.  After this fix, checked to be working in the latest versions of the above browsers plus Firefox.
	» This seems to have been written at a time before the CSS3 gradient spec was finalized.  The ideal solution would be to provide a fix to the skrollr project, but I'll leave that for some other time.
	» Grunt is just sassing at me with “Fatal error: Unable to find local grunt.”, so I guess I'll re-gen `skrollr.min.js` some other time.
Fix: `.color-as-rgb()`/`-hsl()` rounding.

• Wrapped `red`, `green`, and `blue` value code in `round()`.
	» According to the LESS docs red(), green(), and blue() return “integer 0-255”, but the results I'm seeing are full floating-point numbers (after an overlay() or average()).  Looking like a bug in LESS to me.
• Wrapped `hue` value code in `round(…, 3)`.
	» 3 places of precision under a value from `0` to `359.…` should be more than enough, without being excessive.
	» The default looks to be 8 places, which is way too much to be useful, and clumsy for the skrollr code.
• Wrapped `saturation` and `value` value code in `round(…, 2)`.
	» 2 places of precision under a value from `0` to `100` should be more than enough, without being excessive.
	» The default looks to be 8 places (see above).
Sky gradients

• New Skrollr-blending `linear-gradient`s on sky-phase's `.back.fill`.
	» Up-front call to `.linear-gradient()` mixin (see below) with the initial `@sky-eve-*-as-rgb` colors.
	» `-skrollr-dry-animation() {}` call with:
		◦ a `_sky_phase_start` keyframe that provides a plain-W3C-CSS `linear-gradient` with the same “initial” `@sky-eve-*-as-rgb` stops
		◦ and a `_sky_phase_end` keyframe with a `linear-gradient` with `@sky-night-*-as-rgb` stops
	» I like the blending between `-as-rgb` colors better than `-as-hsl` colors.  The former is duller, and looks more like a real sunset, where the latter is just a bit too unnaturally vibrant.
	• `background-color` fallbacks set to the `-mid` color.
	• Sky's text `color` changed to opaque white, and given a soft (3px) 50% opaque black soft shadow, since that's a trick to make text contrast a little better.
• Added a `.linear-gradient()` mixin because Bootstap's `.vertical()` wasn't cutting it.
	» Bootstrap's doesn't support 3 colors… which I now see it has `.vertical-three-colors()` for… but it also doesn't support 4+ colors or any direction.
	» This simpler mixin mostly just passes stuff through, so any `@dir` and `@stops` will be taken as written.
	• `#ambitions-phase` and #title-phase` both switched to use `.linear-gradient` instead of Bootstrap's `#gradient > .vertical()`
Tweak: Blue-tinted `@sky-night-mid` by 40°

• `spin()`ed (hue-shifted) `@sky-night-mid` from its `@sky-night-mid-pure` source -40° to make it bluer and fit better between the other two.
	» Instead of `multiply()`-ing it.  It was dark enough already; multiplying made it too dark.