Update docs, anticipating upcoming testnet changes

[ttmc]

• In the near future, the BigchainDB Testnet won't be at test.bigchaindb.com any more, so just remove all specific mentions of that subdomain.
• In the near future, the BigchainDB Testnet won't require the app_id and app_key headers. It won't require any headers. Therefore keep some docs about how to include specific headers, but don't mention the BigchainDB Testnet.
• Make the version of Sphinx in docs/requirements.txt consistent with the same file in our other repos.
• Assorted formatting and copy-editing of the JavaScript driver docs.

[ttmc]

I also updated the examples in the examples/src/ directory.

[ttmc]

Even though the Travis tests are (or were) failing, we know why, and the root cause was fixed when https://github.com/bigchaindb/bigchaindb/pull/2567 was merged. I'm not sure when that fix will propagate to the tests in this repo.

Therefore I'm going to merge this pull request now, before I forget about it.