- Fixed bug where the overall column width was shadowed by a parameter
  name.
- Removed the (unnecessary) second parameter to column(). The
  value was used to calculate the ratio of $total-width/$calculated-width
  for fluid-width purposes, and therefore should always use the global
  $column count.

The gridsystem-ratio() function calculates the ratio of the
user-specified $total-width, to the calculated grid width.

This ratio was used in every width calculation, and rewriting the
calculations using this gridsystem-ratio() made their purposes much more
obvious.

gridsystem-width() already calculates the width with the outer margin
included. Adding the gutter again makes the width of the row too large.

An expression in the column width calculation was mathematically
equivalent to the value calculated by gridsystem-width(), so I replaced
it with a call to that function.

Just as in the width calculation for column(), the push and pull mixins
contained an expression that was mathematically equivalent to the value
calculated by gridsystem-width(), so I replaced it with a call to that
function.

The previous formula for calculating push/pull margins was incorrect.
The correct behavior is for the push/pull margin of $x to be equivalent
to the width of column($x) plus half of the gutter.

The formula could be simplified further, but I feel that writing it this
way emphesises the relationship to the column width formula.

It didn't take changing default values into account. I updated it to
recalculate the "default width" every time.

- Using 960.gs as a benchmark for what the column widths that are
  generated should be.
- Ensure that the row widths are calculated correctly in relation to the
  column widths.