Add profiling/performance testing tools and track with asv

[keflavich]

spectral-cube is all about performance, so we should be tracking it.

[ChrisBeaumont]

Good point. This could be tricky with the memory mapping, since the OS does a lot of caching that makes repeated runs of a benchmark hard to compare directly. One way to get more repeatable runs is to run a command like purge on OSX to clear the cache, though it's kind of overkill