Add peak interpolation to PitchYinFFT algorithm

I added peak interpolation to YIN since its was not able to detect subtle changes in pitch and interpolation is already contemplated on the original YIN paper.
Peak interpolation is computed with Essentia's standard algorithm PeakDetection.
It has been introduced with a configurable parameter to maintain backwards compatibility. I am realising now that I activated it by default, and maybe it should be off by default.
Please, check the code since it is the first modification I do in Essentia (not still comfortable with the internals) and my knowledge of C++ is more really C. Thanks!

Screenshot that shows the different results :)
![screen shot 2014-01-31 at 18 52 49](https://f.cloud.github.com/assets/34079/2053165/ed01ea64-8aa2-11e3-938d-175339625035.png)