cv.getTextSize bug

the code had an empty std::vector declared, and then passed the raw data pointer to cv::getTextSize. The function [expects](https://github.com/Itseez/opencv/blob/master/modules/core/src/drawing.cpp#L2007) a pointer to a int scalar to fill.

I also made some minor changes, updated the Markdown, and added an example adapted from the OpenCV [docs](http://docs.opencv.org/modules/core/doc/drawing_functions.html#gettextsize)