create a “dotted” style for lineGraphView

Hi, 
For my own use, i needed to have dashed line graph. So i'm pull-requesting it.

To have a dotted line, you need to call the function `gvStyle.setDotted(int length, int space)` on a graphviewstyle element.

Technically, i need to activate hardware acceleration because `DashPathEffect/drawLine` function seems to require it (or is buggy) for recent Android version. The alternative of this would to draw linegraphview with a `canvas.drawPath` and not drawLine, but require a `STROKE` paint style, and can maybe introduce side effects bugs in the library.

There a lot of case that a dotted line can be usefull, like data projection or threshold type like stock value, or bank account value. Here some examples : 
![dotted_graph_1](https://cloud.githubusercontent.com/assets/1333761/3941983/ae6d258e-254d-11e4-98f4-1c346c0d2e97.jpg)
![dotted_graph_2](https://cloud.githubusercontent.com/assets/1333761/3941982/ae6b62a8-254d-11e4-8953-8bb93f38a98c.jpg)