Expand use and function of zen-grid-item-width()

[jstoller]

This branch makes the following changes:

1. The box sizing method check, and corresponding width adjustment, is moved inside the zen-grid-item-width() function itself.
2. $column-position, $gutter-width and $box-sizing variables are added to the zen-grid-item-width() function's input. The gutter-width and $box-sizing variables are needed for the box sizing method check. The column-position variable is added to enable more advanced features in the future, like grids with variable width columns.
3. zen-grid-item-width() is now used to calculate the width in the zen-grid-item mixin, as well as the zen-grid-flow-item mixin. It is also used to calculate the margins for grid items.
4. If zen-grid-item-width() is called without any variables, it will now return the full grid width by default.

In addition to making the zen-grid-item-width() function more useful in its own right, which may have uses outside of the zen-grid mixins themselves, these updates open the door to more seamless future enhancements. For instance, you can now implement a non-equal grid columns by expanding zen-grid-item-width(), without negatively impacting anyone already using this plugin.

[JohnAlbin]

Addressing your points:

1. Absolutely. I wish I had seen these commits before I did that bug fix at bfe88ec. I would have your commit to give you credit. :-)
2. I agree with you on 2 of the variables. Not sure I see the necessity for $column-position yet.
3. Yep. That was done in bfe88ec as well.
4. I don't understand why this is needed. $zen-grid-width already has the full width of the grid.