Remove polymer_ui_elements dependency

I believe that this was causing a circular dependency since
polymer_ui_elements also depended on this package. The end result (at
least for me locally) was that pub serve would not serve resources
from either package.

More info at: http://japhr.blogspot.com/2013/12/pub-server-polymer-elements.html

Also, should polymer_ui_elements be dependent on this package? It 
seems like they ought to be completely independent of each other.