Option to specify whether labels are displayed

Implemented an option to omit displaying labels. This is useful when you have to display many (thousands in my case) events and what matters is the overall image and not the fine details. See below for the visual difference:

**display.label = T**
![display label](https://cloud.githubusercontent.com/assets/386172/4751034/8bab9c7a-5a99-11e4-8e7a-304758a878b6.png)

**display.label = F**
![no display label](https://cloud.githubusercontent.com/assets/386172/4751031/83d4fea6-5a99-11e4-9c99-8d3d930cbdb0.png)