Take RGB to sRGB conversion matrix from RAW instead of hardcoded one.

[brotherofken]

Color convertion matrix hardcoded into srgb function will result in wrong white balance for almost all cameras other than Canon 5D III that Timothy was used in his work.

We should instead use camera's auto white balance that provided in imgdata.color.rgb_cam field.

For rare cases when this field is not available for some reason it's probably it's worth to implement some white balance method (e.g. gray world, convolutional color constancy).

This issue related to #36

[brotherofken]

Closed due to #54