ENH: add runbenchmarks.py

Add a script to run benchmarks.  This is analogous to the script that runs tests.

In its current form, it awkwardly copies the function `build_project` from `runtests.py`, but because of the nature of these development script I don't know the best way to make them share code.

An alternative design would put the benchmark-running functionality into `runtests.py`.  Maybe this would be a better decision.  In the current PR, the benchmark runner has three modes:
1) run all the scipy benchmarks using just `runbenchmarks.py`
2) run the benchmarks for a submodule like `runbenchmarks.py -s linalg`
3) run a specific benchmark like `runbenchmarks.py -b scipy/linalg/benchmarks/bench_decomp.py:bench_eigvals`
I'm not sure how to add these modes into `runtests.py` without using awkward command line parameter names.