Benchmark Python CIELAB conversion pipeline

[dzhoshkun]

Virtually the same code as #11, but using e.g. PyCuda.

[dzhoshkun]

Preliminary roadmap:

• [ ] factor I420-to-CIELAB conversion out to kernel
• [ ] factor CIELAB-to-I420 conversion out to kernel
• [ ] read image using e.g. scikit-image directly into I420
• [ ] refactor above two kernels to use device memory allocated using PyCuda