Don't need to re-compute already computed statement nodes
Fix ImportError with OpenCL
Turn on debugging again and update external viennacl-dev
Fix cache error
Stub out CustomNode class
Add CustomNode handling to Statement
Fix bugs; add simple CustomNode example; begin implementing CustomNode
Add proof-of-concept CustomNode.execute -- segfaults for now...
Fix segfault
Update TODO list
Generalise CustomNode to other backends; support precompiled pyopencl.Kernel instances
Improve opencl kernel operand handling; add matrix example to custom node example
Reorganise example for clarity