Adding support for creating fluid dynamic grid

Hi,

With the objective to create fluid *dynamic* grid with Sass Bootstrap I patched the code (a really small patch... only adding !important behind mixins) so that now it is really easy to change dynamically the grid structure, [here is a demo](https://github.com/barraq/sass-twitter-bootstrap/tree/master/demos/fluid-grid-demo-1) with the output [JSFiddle](http://jsfiddle.net/gh/get/mootools/1.4.5/barraq/sass-twitter-bootstrap/tree/master/demos/fluid-grid-demo-1/) (i was unable to load the scss file within jsfiddle...)

Basically the demo.scss looks like:

```scss
@import "../../lib/bootstrap.scss";
@import "../../lib/responsive.scss";

// body is the fluid containter
body { @extend .container-fluid; }

// make elements "column fluid"
#page-body, 
#page-sidebar {
    @extend [class*="span"];
}
section:not(.main) > section {
    @extend [class*="span"];
    @media (max-width: 1199px) { margin-left: 0 !important; }
}

// make elements "row fluid"
#page,
#page > section:not(.main) {
    @extend .row-fluid;
}

// responsive grid
@media (min-width: 768px) and (max-width: 979px) {
    #page-body { @include grid-fluid-span(7, $fluidGridColumnWidth768, $fluidGridGutterWidth768); }
    #page-sidebar { @include grid-fluid-span(5, $fluidGridColumnWidth768, $fluidGridGutterWidth768); }
}

@media (min-width: 980px) and (max-width: 1199px) {
    #page-body { @include grid-fluid-span(7, $fluidGridColumnWidth, $fluidGridGutterWidth); }
    #page-sidebar { @include grid-fluid-span(5, $fluidGridColumnWidth, $fluidGridGutterWidth); }
}

@media (min-width: 1200px) {
    section:not(.main) > section { @include grid-fluid-span(6, $fluidGridColumnWidth1200, $fluidGridGutterWidth1200); }
    #page-body { @include grid-fluid-span(8, $fluidGridColumnWidth1200, $fluidGridGutterWidth1200); }
    #page-sidebar { @include grid-fluid-span(4, $fluidGridColumnWidth1200, $fluidGridGutterWidth1200); }
}
```

The only file changed is the _mixins.scss.
The other files in the patch are just the demo and README.

If there is a better approach to create fluid dynamic grid with Sass Bootstrap let me know!

Thanks