Add absolutePanel and fixedPanel (and jquery-ui)

jQueryUI is needed for draggable functionality.
Smarter cursor choosing for absolutePanel
Minor doc tweak
Merge remote-tracking branch 'origin/master' into feature/absolute-panel

Conflicts:
	DESCRIPTION
Unify jqueryui copies
fixes #90: textOuput() can be put in any tag now

for the example, we use <span>, which is allowed in <h3>
Define a non-S3 input registry.

Revises the approach taken in #233.

!! No longer casts NULLs in incoming JSON to NA.
Update inputHandler docs, revamped tests.
Removed deprecated function documentation.
Added tests, stop converting NULLs to NA.
Flush react/output onSessionEnd
Merge pull request #370 from trestletech/flush-on-session-end

Flush react/output onSessionEnd
Merge pull request #369 from trestletech/input-handler-patch

Define a non-S3 input registry.
Don't attempt to write if the connection is closed.
Merge pull request #371 from trestletech/no-write-closed

Don't attempt to write if the connection is closed.
Bind both mouse and touch events for jslider

This brings in commit 468002a for jslider. It fixes the problem where
dragging the slider didn't work in the RStudio viewer pane.
reactive(NULL) works now
and reactive({}) also works; closes #366
Missing semicolon
Add new object for handling singletons