Change AppCache from browser whitelist to blacklist
History update
Merge branch 'pr-2241' into devel
Consider empty passwords to be a user error

Programming errors like "the code doesn't try to provide a password"
should throw, but user errors like "the code pulled a password from an
input but it was empty" should use the callback.

Fixes #2272.
Move spiderable phantom script to an asset
Fix Markdown

Fix Markdown in History.md
Fix {{#each todos}} rendering when !list_id

{{#each todos}} rendering was broken when no list was selected, since {{#each}} cannot iterate on "{}" (it only takes lists and falsey values).
This code path is never hit in the todos app as it is shipped, but it can cause problems with people modifying the example code for their own apps.
Facilitate debugging METEOR_SETTINGS