Fix dropdown display in style guide; fix rendering of options where...

... 'selected' incorrectly applied to all.

Style guide CSS:
Before:
![screen shot 2014-06-09 at 2 28 59 pm](https://cloud.githubusercontent.com/assets/3247835/3222893/63360254-f01c-11e3-8ef9-5cf14d390d2e.png)
After:
![screen shot 2014-06-09 at 2 29 13 pm](https://cloud.githubusercontent.com/assets/3247835/3222941/e80e3906-f01c-11e3-8a22-05641ebadf67.png)

Dropdown 'selected' bug:
Needed to add a check for "selected" before the boolean comparison when generating the option tags. When no 'selected' property exists as a property of the field the code was always returning a 'true' result and marking every option as selected.

Multiple Select input widths:
Added a 'dropdown_width' property that allows a custom percentage width to override the default width settings if you don't want each input to be the same length. Helped with balancing the look of the style guide example. 
I think I would probably scope this to just the dropdowns page. Just in case we include any actual form elements in other parts of the style guide. `.styleguide-block__item--#{i}` is used on most pages