Media objects: update class name convention

Includes the removal of `@extend` for flex-embeds
Update text size classes

Remove previous h1 class names that tied the styles to a specific
context (e.g. 'feature'), and add text classes for smaller and
larger instances of copy.

Use `@extend` to combine selectors for headings.
Buttons: adjust alignment of btn-group

Since they are inline-block, they leave excess space beneath them.
Setting the vertical alignment to `middle` hides this space and is
likely to me a more commonly desired style too.
Start breaking docs into more sections

Add 'core' and 'ui' sections to contain the documentation for various
sections of the toolkit. Currently in rough placeholder state.

Component section will need the same treatment. JavaScript section might
shift towards more standalone JS, e.g., the script optimisations, any
polyfills, and maybe an overview of the JS conventions used. The
UI-based JS will probably be covered when discussing components, since
it doesn't make much sense to separately present JS that is bound to a
component.

Philosophy could get moved into the Wiki. Need example sites/apps to
help demonstrate ways to combine the various HTML/CSS/JS templates and
components.
Split components section into multiple docs
Ensure alignment of grids is always known

Explicitly set the `text-align` on the grids to avoid chance of the
alignment being affected if they are the child of an element which
modifies the text alignment. The default value can be toggled in the
SASS variables to accomodate rtl.
Update grid docs

Improve the layout, explanations, and examples
Minor update to variables

Begin cleaning up the variables so that the color palette acts as the
single place where any colors are set. Those variables can then be used
in more abstract variables that then go on to populate the rest of the
SCSS. That way the color palette of a site can be fully customised from
the `_variables.scss` file without variable names elsewhere in the
document no longer making sense.

Same principle for font families.
Tweaks and design update to typography docs

Improve the appearance of the typography docs.
Make some small adjustments to the base typography and switch some
hard-coded values for variables.
Grid: add fluid offsets

Fluid offsets provided via prepending a percentage margin. This is
bundled within the function used to create grid units.
Add em-based site width and other tweaks

A new variable converts a pixel-based site `max-width` to an em-based
one. This can be used to retain certain typographic proportions at
different base font sizes.

Add a new about the alternative grid whitespace-collapsing method to
note how it will break the grid if combined with an em-based grid gutter
width.

Simplify the pre-formatted text's monospace font stack to use a one-
letter fallback font. It's all that's needed to trigger the bug-fixing
behaviour (correct font sizing).
Further typography and base adjustments

Explicitly reference the typographic scale being used in the CSS
comments of the base file. Adjust the heading styles and classes to
stick to the scale. Edit docs to use the new `h1-l` class and reserve
the `h1-xl` class for very large text in "hero"-type contexts.

Other minor fixes to the base normalizations.
Add space helper classes

Use the same naming pattern that OOCSS uses. Widths of the padding and
margin space are controlled via variables.
Rename project
Update README
Switch back to bidirectional grid margins

Makes it easier to add borders between grid units.