all: Add GPU mining, disabled by default

Godeps/Godeps.json

...kspace/src/github.com/Gustav-Simonsson/go-opencl/cl/cl.go

...e/src/github.com/Gustav-Simonsson/go-opencl/cl/cl_test.go

...e/src/github.com/Gustav-Simonsson/go-opencl/cl/context.go

...ce/src/github.com/Gustav-Simonsson/go-opencl/cl/device.go

.../src/github.com/Gustav-Simonsson/go-opencl/cl/device12.go

...github.com/Gustav-Simonsson/go-opencl/cl/headers/1.2/cl.h

...ub.com/Gustav-Simonsson/go-opencl/cl/headers/1.2/cl_ext.h

...hub.com/Gustav-Simonsson/go-opencl/cl/headers/1.2/cl_gl.h

...com/Gustav-Simonsson/go-opencl/cl/headers/1.2/cl_gl_ext.h

...m/Gustav-Simonsson/go-opencl/cl/headers/1.2/cl_platform.h

...ub.com/Gustav-Simonsson/go-opencl/cl/headers/1.2/opencl.h

...ace/src/github.com/Gustav-Simonsson/go-opencl/cl/image.go

...ce/src/github.com/Gustav-Simonsson/go-opencl/cl/kernel.go

.../src/github.com/Gustav-Simonsson/go-opencl/cl/kernel10.go

.../src/github.com/Gustav-Simonsson/go-opencl/cl/kernel12.go

.../src/github.com/Gustav-Simonsson/go-opencl/cl/platform.go

...e/src/github.com/Gustav-Simonsson/go-opencl/cl/program.go

...ace/src/github.com/Gustav-Simonsson/go-opencl/cl/queue.go

...ace/src/github.com/Gustav-Simonsson/go-opencl/cl/types.go