Add Performance Benchmarking

[Andrew Hawker]

I did some very basic work with pytest-benchmark during development. However, a more complete and robust set of performance tests for common API calls/flows should be written.

Completeness Criteria:

• New benchmark module, say test_performance.py.
• Use pytest groups on the module or filter it out so make test doesn't always run it.
• Add make benchmark or some similar target to execute them.
• Add as a new tox and Travis CI target.
• Pick a stable machine to run benchmarks and add a baseline to the README.

[Andrew Hawker]

Initial scaffolding and rudimentary benchmarks added in #10