Set margins to 0 for `a` & `label` elements in dropdown-menu

Override any existing margin rules for `a` and `label` elements
inside dropdown-menus. For example, if you have a nav bar with
`a` elements spaced out using margins, any dropdown menus would
still display menu items without margins.
Merge pull request #58 from dpbus/master

Set margins to 0 for `a` & `label` elements in dropdown-menu