ZEPPELIN-162 shadow for paragraph instead of border

[ZEPPELIN-162](https://zeppelin-project.atlassian.net/browse/ZEPPELIN-162)
@anthonycorbacho mentioned sometimes it's hard to recognize paragraph's dimension at #106 
One of the reason i think is, on mouse over paragraph, border appears, but the color and width of border is too light.

I changed it to shadow, like screenshots below

![image](https://cloud.githubusercontent.com/assets/1540981/4372198/bdb1db4c-4321-11e4-8216-be892ef02f44.png)

Looks better? ready to go.