Create ipad-above mixin.

This mixin defines the media query for all screens above 768px, which previously didn't exist.
Bugfix: Make %row extend %clear instead of .clear.

Previously, %row would not clear as expected if $classes was false.
Create helper mixins

- Users of groundwork looking for semantic implementations can now include instead of extend helpers (e.g. "@include _pad-left;"). This is particularly useful for media queries - at the moment extends within @media queries don't work the way you would expect them to, but mixins do. There are now 3 ways to utilise layout helpers - as mixins, as placeholders, and as classes.
- mixins have been given an _underscore prefix, because of uncertainty regarding name clashes and potential for confusion (e.g. "border-bottom" could be both a CSS property and a mixin - this may or may not be considered an issue). Make a decision as to whether to keep or axe the prefix. The -dash prefix is also worth considering.
- It'll be good to have a 3-column table somewhere on a page saying "hey guys, you can activate a layout helper in 3 ways. E.g. _pull-right, %pull-right, and .pull-right" and so on.
Merge branch 'master' into helper-mixins

Conflicts:
	src/scss/_helpers.scss

Seems like truncate is a new thing. Added it in.
Restructure tests folder

Tests folder is now structured for test-driven development if any dev wants to develop that way. Each test consists of a simple page + CSS generated with Jade/SCSS. Note that for space reasons, compiled CSS is not included, you have to compile tests/src/scss/(whatever).scss into tests/(whatever).css locally.

Did up a first test for grids, which expands on whatever was in the test folder before this. Looks good - seems like grid divisions, re-ordering, offsetting, and centering work as expected for extends. Mobile adjustments aren't extend-ready yet - someone should work on this in future.
Add test for helpers to the tests folder

- This was done in order to test upcoming helper-mixin functionality (currently unmerged in a separate branch).
- Looking at the test, it seems like extends are working the same as their class counterparts. Will update the test for includes once this branch is merged with helper-mixins.
- rotate-90s and rotate-270s overflow around their containers. Is this a bug or is the test usage of them wrong or what? To be looked at in future.
- Removed obsolete test.jade (Forgot to remove this in the last commit).
Merge branch 'tests' into helper-mixins
Update test for helpers with tests for includes

- Now that helpers-mixins and tests have been merged, updated tests to check that helper-mixins are working correctly. They seem to be.
- "truncate" seems missed out from the class list for generating classes. Added it.
Merge branch 'master' into helper-mixins
Merge based on the latest upstream commit

Take note:
- Disabled added in to helpers.
- Removed test.css (because we have test-helpers.scss and test-grid.scss)
- Recompiled groundwork.css.
added Aria role support to inline modal dialogs
added Modernizr touch feature detection to menus component
added Aria role support for tabs, fixed disabled element hover styles bug
improved disabled styles and functionality on nav, tabs & buttons, fixed left border on nav submenu elements, enhanced keyboard accessibility, added keyboard accessibility for 'Hamburger' menus
Merge pull request #72 from xinix/master

Tile component
extended disabled state to apply to form elements, fixed binding on invalid/error state form elements
enhanced newly added tiles components
fixed up navigation hover/click functionality
improved tile component styles
fixed margins/paddings on hamburger menu navigation, changed row to extend placeholder clear class