Simplify imgproc class.

[kalwalt]

As stated in this comment i will try to simplify and improve the imgproc class. At this time the methods aretoo Emscripten orientedin the sense they are specifically designed to be compiled directly with EMscripten, but if you want to use them internally with C++ code (and then after compiled with Emscripten) it can be hard or worste ca scenario near impossible. I will try to simplify the grayscale's methods and then if i can others. This is a necessary step for PR #31.

List of changes:

• simplify grayscale's methods.
• remove if else for Emscripten grayscale, instead add if define only for the Emscripten version.
• improve and simplify pyrdown.
• simplified warp.
• simplified resample.
• use templating as possible.

[kalwalt]

I fixed the Imgproc grayscale binding, but i noticed that pyrdwon example create only black pyramids. Need to see where it's broken, probably one of latest changes.

[kalwalt]

It seems strange to me that this can not copy the data, i will fill an issue for this.