Add JavaScript to enable the use of PrimaryLists

One of the features implemented in [govspeak][1] is to allow long lists
to be collapsed to only show the first `n` items. This JavaScript makes
the frontend behaviour possible.

[1]: https://github.com/alphagov/govspeak
Merge pull request #93 from alphagov/primary-links

Add JavaScript to enable the use of PrimaryLists
Add stick at top when scrolling JavaScript

This lets you have elements that scroll with the page as you scroll.
This has been ported from the whitehall codebase but the tests updated
to use Jasmine rather than jUnit.
Trigger pageSizeChanged on show and hide links

So that the sticky scripts work in the correct place fire page size
changed on showing and hiding the primary link code.
Merge pull request #94 from alphagov/sticky

Add stick at top when scrolling JavaScript
Add a file-path function for agnostic image urls

The `file-path` function will return either a url prefixed with the
contents of the `path` variable if it is set, or a `image-url` if it is
not set (the default). This lets you use the external link icons on
projects that only use vanilla Sass.
Merge pull request #96 from alphagov/file-url

Add a file-path function for agnostic image urls
Fix bug where button mixin requires colours include
Merge pull request #97 from alphagov/button-bugfix

Fix bug where button mixin requires @import '_colours';
Add sass-cache directories to gitignore
Remove Ruby specific syntax from filter attribute

Using #{ruby_variable_interpolation} means that this file can't be
compiled using node-sass.

Change the value to use Sass arithmetic, string concatenation and
the unquote method to avoid this Ruby specific syntax.

http://sass-lang.com/documentation/Sass/Script/Functions.html#unquote-instance_method
Merge pull request #98 from alphagov/remove-ruby-syntax-filter-opacity

Remove Ruby specific syntax from filter attribute