include ExecutableCTarget in main __init__ and docs

Just a bit of cleanup here -- include the ExecutableCTarget in init.py such that it can be accessed via from loopy import ExecutableCTarget, include it in the autoclass docs, and finally check for an executable target in the CompiledCKernel (as the CKernelExecutor is only instantiated from ExecutableCTarget, although we could just eliminate this assert and assume it to be true).

This last one is more of a directional question -- do we want to make a physical distinction between executable targets and codegen targets? This is already the case for PyOpenCLTarget and OpenCLTarget (and now the base CTarget \ ExecutableCTarget) -- in the future I would extend this to, e.g., ISPC.