Adds style options to select gnuplot data style

When plotting dense points, lines between each datapoint can make the
output quite cumbersome. This adds the possibility to display only
points, and two additional styles as a bonus (dot and circle).

The HTTP API now handles the `&style` parameter as well to inject any
additional style.


![tsdb-styles-1](https://cloud.githubusercontent.com/assets/231014/4516008/85f7aa92-4bdc-11e4-9894-40a8e7e350ca.jpg)

![tsdb-styles-2](https://cloud.githubusercontent.com/assets/231014/4516010/8c00e20a-4bdc-11e4-8265-6d228a4c9f95.jpg)