adding several improvements to the visual design

Feel free to criticize :)
1. mouse over on the history graph is linked only to rectangles
2. changing the mouse icon over text in the forcemap layout
3. using rounded rectangles in forcemap

http://10.1.98.108:8080/~jbeezley/twitter-mention-history/

Several interactions involving clicking on things still need to be fixed.  I'm looking into them.