Peak Hold 

This branch adds peak hold as of bug #73.
Peaks are displayed with the same color of FFT but with a lower alpha value.

Peaks are computed using the points of the FFT graph so when the plotter area changes (e.g. the window is resized) peaks get invalidated and are reset. If this behavior is not ok I will modify the code to keep track of the peaks using the incoming FFT data. This will require re-mapping data to pixel heights (i.e. another call to getScreenIntegerFFTData).

![screenshot from 2013-10-25 00 35 27](https://f.cloud.github.com/assets/5714076/1404423/a90e7be0-3d06-11e3-80c9-dfec4d97eadd.png)