show which device is in use in benchmark
fix memory leak in OpenCL implementation

The lowpass, subbands and scales properties of
backend_opencl.TransformDomainSignal were indeed memoized but memoized
globally meaning that referencing them would keep a reference to the
TransformDomainSignal object indefinitely leading to memory growing
without bound. Replace the @memoize decorator with a simple
attribute-based cache.
make benchmark script not fail on non-OpenCL machines

Skip the benchmark all together if there is not OpenCL present.
Merge pull request #40 from rjw57/opencl-bugfix

fixes for OpenCL memory leak