Add routing capabilities

[michelmansour]

• Include react-router as a dependency to enable client-side routing
• Move full poem display to new client route (/poem/:poemId)
• Modify main poem list to show only poem titles and authors
• Add nav bar to allow navigation from a poem back to the list
• Add code to clear polling intervals of poems and comments when navigating away from their respective views
• Add server route to fetch individual poems
• Add catch-all route to server to support use of browser history API
• Rename some React components to better reflect their purpose

[drewrey]

This looks good to me.