* Add start and end support to the horizontal gradient as well
* Change all variables from camelCase to use-dashes
* Better comments in the gradients mixins area
* Update current uses of gradients in dropdowns and carousel to specify only two values by direct assignment of variables

refactor some of the carousel caption stuff to center it and tighten styles up; update carousel template to start to redo mobile first

* They're hidden by default in the mobile-first views because otherwise they just cover up the slides
* In mobile they're centered, and then are shown left aligned at larger viewports

* Drop custom type arrow controls for glyphicons icons instead for more consistent rendering
* Remove the captions and instead let folks implement what they want in the carousel
* Move the indicators to the bottom middle instead of top right
* Lighten up gradients on the edges for less harsh lighting

lessified .carousel-caption

Mostly doubling-up :hover styles to also cover :focus, as a first step
to making the framework more keyboard-friendly.
Additionally, fixed two small markup issues in the docs/examples to
make the "Learn more" large primary button-styled links
keyboard-focusable (as without href they're treated as non-tabable
anchors).

adding actual images to carousel to ensure you can see things are animating; updated styles of carousel controls to work on light and dark backgrounds and use opacity for effects