Set default `Accounts.callLoginMethod` argument to `{}`.

This avoids a match failed error and instead gives a more descriptive
"unrecognized options for login request" error. Fixes #2618.
Make end-of-redirect response a blank page.

We think this is better than putting un-customizable, un-formattable
text. See #2625.
Use wrapAsync instead of Future.wrap
api doc: add a working showdown (Markdown) example

I couldn't figure out an easy way to include a Spacebars operator like
{{#markdown}} in a Markdown code block, hence the <code>{&#123;#...
trickery. I think I can clean this up a little with the {{dstache}}
template helper, though. See next commit.
api doc: use {{dstache}} template helper

Less/cleaner source code.