Fixed a bug in YamlOutput that will produce invalid json files, add python test to test validness of generated json files

YamlOutput produced invalid jsons files in the case when unescaped strings were stored.
Add peak interpolation to PitchYinFFT algorithm
Add YIN peak interpolation parameter to Vamp plugin
Add ZeroCrossingRate algorithm to Vamp plugin
Fix code style in PitchYinFFT
Merge pull request #52 from hector/2.0

Add peak interpolation to PitchYinFFT algorithm
fixed allowed interval for magnitudeThreshold param in SpectralPeaks

It was impossible to setup a threshold for values on db scale as they were negative.
Merge branch '2.0' of https://github.com/MTG/essentia into 2.0
update documentation for SpectralPeaks
Added function that tries to fix and strips invalid UTF8 characters when reading tags in MetadataReader
Fix typo in extractor's command-line help text
update documentation with info about high-level models