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
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'
Document UI.getElementData

While doing this, I realized that this function
can be called on text nodes as well, so it's probably
better named UI.getNodeData?
Add credentialSecret to `Google.retrieveCredential`

I missed this one in 0fa591bc5.
Fix accidentally-hardcoded Twitter URL in oauth1

Fixes #2154.
Add short aliases for '--add' and '--remove' options in "meteor authorized"

Fixes #2155
Add History item for e86578e5.

Conflicts:
	History.md
Update History
Use underscore in oauth-encryption package
Update History for 1a9509
Add 0.8.1.3 section header to History
Tweak history
Revert "Add short aliases for '--add' and '--remove' options in "meteor authorized""

This reverts commit 32fc06e7f86b5f7b18226aa25d38d25af0461f9e.

As Glasser points out, our command option parsing is such that all `-a`
flags for commands have to be consistent in whether they are boolean or
not. Since we could easily imagine wanting a boolean `-a` or `-r` flag
in the future, we should be careful about adding these aliases. We'll
update the docs to remove `-a` and `-r` instead.