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. 
Is that as simple as moving this to the dropdown.sass partial?