Benchmark Theano meta-optimizer

This adds a benchmark for the brand new Theano convolution meta-optimizer -- an attempt to perform as well as the cherry-picking row (halfway there, cuda-convnet and cuda-convnet2 are not included in the meta-optimizer yet).

Furthermore, it makes the Theano benchmark a little more convenient in that each of the implementations can be turned off via a comma-separated `SKIP` environment variable. And for debugging purposes, it supports a `PRINT_GRAPH` environment variable that prints the Theano graph for each compiled function when set to a nonzero value (I use the benchmark script as a handy development tool, not just for benchmarking).

This time it might be interesting to re-run the benchmark (according to the updated README file; it's `SKIP=legacy` now instead of `SKIP_LEGACY=1` if you want to skip timing the legacy code).