Color "skins" (#1242)

* Add YIQ Color Contrast mixin
* Remove text underline from `.btn` links
* Move YIQ variables
* Simplify button classes using YIQ color contrast mixin
  - DRY up button CSS using Sass list and YIQ color contrast mixin.
  - Move `color` and `background-color` to new `btn--primary` class instead of assinging on the default class. Removes the need to override them.
* Add `.btn--primary` to buttons that just had `.btn`
* Apply changes to `/docs`
* Add `.btn--primary` class
* Update CHANGELOG and history
* Add sample form
* Abstract colors away into Sass variables for easier themeing
* Add "dark" skin
* Replace hardcoded color with SCSS variable
* Invert Font Awesome icons' colors in author sidebar and footer
* Add Sass changes to `/docs`
* Use primary button type instead of inverse
* Add missing `!default` on `$muted-text-color`
* Add `contrast` and `sunrise` skin colors
* Add `dirt` skin color
* Add `air` skin color option
* Add `mint` skin color
* Add `btn--primary` class to Submit Comment button
* Set skin to `default`
* Document skin color options
* Add note about skin SCSS import

Close #1208

docs/_posts/2013-01-11-markup-html-tags-and-formatting.md

@@ -102,6 +102,17 @@ Do It Live
   3. List item three
   4. List item four
 
+## Forms
+
+<form>
+  <fieldset>
+    <legend>Personalia:</legend>
+    Name: <input type="text" size="30"><br>
+    Email: <input type="text" size="30"><br>
+    Date of birth: <input type="text" size="10">
+  </fieldset>
+</form>
+
 ## Buttons
 
 Make any link standout more when applying the `.btn` class.