Luminance differs between Calculate.luminance and Filter.threshold

[Murat Knecht]

Calculate.luminance() calculates a differing luminance than Filter.threshold(), rendering either one of them unusable if used in combination.

https://github.com/meltingice/CamanJS/blob/master/src/core/calculate.coffee#L27
https://github.com/meltingice/CamanJS-Plugins/blob/master/src/threshold.coffee#L3

Please use the former in the latter. Cost me quite a bit of time to notice differing luminance calculations.