Create a webcl.js file to be shared by some demos

issue #12

Some WebCL functions to help with some common or repeated tasks like get context using CPU or GPU, build program and create commandQueue
 
Eg:
 WebCLCommon.init("GPU");
 var ctx = WebCLCommon.createContext();
 var program = WebCLCommon.createProgramBuild(kernelSrc);
 var cmdQueue = WebCLCommon.createCommandQueue();

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