Makes hover detail arrow rendering more reliable

Uses the 'CSS triangle' trick to render the left and right arrows on the hover detail item, rather than using a glyph (whose rendering might change depending on the font used).

The technique is described here:

http://appendto.com/blog/2013/03/pure-css-triangles-explained/

### Left
*Before*
![left-before](https://f.cloud.github.com/assets/1253708/1348991/24c69c12-36dc-11e3-8515-1e7b7a990661.png)

*After*
![left-after](https://f.cloud.github.com/assets/1253708/1348987/1160677a-36dc-11e3-9577-f4f218285222.png)

### Right
*Before*
![right-before](https://f.cloud.github.com/assets/1253708/1348989/2113fcb8-36dc-11e3-9f39-7ed59c5d3efc.png)

*After*
![right-after](https://f.cloud.github.com/assets/1253708/1348988/18e3ea12-36dc-11e3-9b28-736260bb778e.png)