Add LHCb plot style.

This commit adds an LHCb plot style based on one of several versions of the official style in distribution.

![lhcb_style](https://f.cloud.github.com/assets/2175510/347270/67714ba4-9eb9-11e2-8a79-447f463f7825.png)

The y-axis title of the test plot is clipped due to the height of the derivative, but I've left it as is for consistency with the ATLAS and CMS examples.