This pull request adds configuration and test files for the airspeed velocity performance benchmarking tool.

This is new to me, but I'm putting it up here for feedback. I developed this while benchmarking #528 and #529. It seems useful. I'm not sure what the standard should be for merging this.

@mikofski do you have any experience using asv or similar tools? Anyone else?

In the long run, it would be ideal to set up a cloud service to automatically run these tests on the same (possibly virtual) hardware each time. As of now, I only want to consider running these tests locally.

• Closes Add a performance benchmarking tool #419
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes git diff upstream/master -u -- "*.py" | flake8 --diff
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

other:

• add a simple readme like scipy's -- rely on asv's documentation for all but the most basic commands and instructions
• consider benchmarks/benchmarks rather than asv_bench/benchmarks (copied from xarray)