Add README badges, branch coverage

[bollwyvl]

• follow-on to Add codecov and coverage threshold, hoist test requirements to setup.py #348
• mostly just want to see if we start getting the codecov PR comment, and if i can fix it with what perms i have...
• adds the codecov badge (and friends) to README with links to various sources
• enables branch coverage, which is slightly more pedantic about whether, e.g. a function with two separate if..elif..else statements executes the permutations of the full set

[bollwyvl]

Hm, the docs just talk about turning the PR comments off, or configuring it, not how to turn it on...

[bollwyvl]

Ah, i had to turn on the app. I did so (because I could), but we can turn it off if we don't like it (or like having codecov have write access to PRs). Anyhow, we should at least see if the in-PR report will be useful.

The other approach is to have a dedicated oauth bot.

[bollwyvl]

Hm, just kidding, no report for us. Well, at least we got badges. I guess we leave the app connected on for a sec, maybe it would start with new PRs? I'll try reopening this one...