File per export

[jrdnbradford]

Closes #67. Between this and work done in #68, I think this gets the export/test structure we want, or at least much closer given the general fuzziness of such decisions.

• documented gutenberg_add_metadata
• created a test-data.R file and moved data-related tests there
• renamed test-metadata.R to test-gutenberg_works.R

Now each exported group of functions has a corresponding test file.

[jonthegeek]

Thank you again! Hopefully I can finish what I'm working on and we can sort all of this out into a release! I might end up doing that this week anyway if it doesn't look like gutenbergr.data is going to wrap up soon.