added a nav bar and placeholder links

I modifed the application.html.erb and added a partial called _navigation.html.erb in order to provide a nav bar to the page.  There is a "Random word" link on the left, and two placeholder links on the right with no assigned routes yet.  They can be utilized to add a "Search" and a "Request a Word" functions, or they can be removed entirely if we don't get to that.  

Comments?  Shall we merge this?