Fix 0.8.1 regression in ROOT_URL with path

Fixes #2109.
Add credential secret argument to `retrieveCredential` functions.

Fixes #2118.
Unregister sessions from server when their heartbeats time out.

Conflicts:
	History.md
Update History.md
Make a History sentence more sentence-y
docs: turn off appcache on safari

Turns out AppCache on Safari 7 is totally broken (see
http://stackoverflow.com/questions/22888945/safari-7-application-cache-does-not-work).
This, combined with our "reload if you can't load CSS or JS" strategy for
multi-server deploy causes infinite reloads in some cases.

Hopefully this will resolve the sporadic complaints we get
from some users about infinite reloads.
Update banner and notices
Update docs to 0.8.1.2
Update examples to 0.8.1.2
Merge branch 'release-0.8.1.2'
Merge branch 'master' into devel