TST: Add format, transformer, and type tests to FeatureData

[jakereps]

Committing by test type to allow looking at individual package diffs.

Resolves #42

[jakereps]

Tests will fail until qiime2/qiime2/pull/152 is merged.

[jakereps]

I'll revisit the transformer tests on this one tomorrow, because they are really gross. FeatureTable went along a little more nicely, but all the background df magic in this one makes it hard to compare ins and outs on the tests without mimicking the magic leading to tons of hideous code.

[thermokarst]

@jakereps: is this ready for review now? No rush either way, just didn't want to review if you weren't ready yet. Thanks!

[jakereps]

The only part that will most likely change or have to be changed if it is reviewed now is the transformer tests. Is it ok to import the helpers used within the transformers to clean things up a bit? There is some black magic going on behind the scenes with the data frames, and having to replicate it for every test adds a ridiculous amount of redundant code (I think I stripped a bunch out on my last reviewing and just had it validate the returned type on some)

[jakereps]

@thermokarst, I took a swing at semi, hopefully, maybe, cleaning up the transformer tests, so this should be ready for review!

[thermokarst]

This looks great, thanks @jakereps!