Introduce enqueue_copy().

doc/source/index.rst

doc/source/misc.rst

doc/source/runtime.rst

examples/demo.py

examples/demo_elementwise_complex.py

examples/matrix-multiply.py

pyopencl/__init__.py

pyopencl/array.py

src/wrapper/wrap_cl_part_1.cpp

src/wrapper/wrap_cl_part_2.cpp