Mixin defaults ...

Changes:

1. SCSS test file.
2. Added `.sass-cache*` to `.gitignore`.
3. Added mixin default args.
4. Cleaned up comments (wrapping to 72 characters).
5. Added property descriptions.
6. Added link to W3.org docs.
7. Slighty modified logic for a few `@if ...` directives.
Merge pull request #6 from mhulse/patch-defaults

Mixin defaults ...
Change flexbox and inline-flex to a class

Changing flexbox and inline-flex from a mixin to a class should produce less CSS when compiled.
Update tests.scss to reflect core changes

Using @extend rather than calling a mixin.
Merge pull request #8 from tjFogarty/master

Updated to produce less CSS when compiled
Placeholder + Mixin Harmony

Revert to mixins, but add placeholders too.
Fixed the reference url for flex-basis
Merge pull request #9 from paradox41/patch-1

Fixed the reference url for flex-basis
Check if the argument passed to flex() is a list and return the right value for -webkit-box-flex. Fixes #10.