Avoid conflict with other selects when checking for value presence

The check in line 88 for an <option /> tag with the same value being added can potentially result in a false positive if there is another <select/> element on the page with the same value on an option. Adding the context to the jquery expression seems to fix it.
Merge pull request #33 from oarevalo/patch-1

Avoid conflict with other selects when checking for value presence