Revert "Return null if treeItemChildren length is 0."

This reverts commit ca545666b76fbaa30a5e19f7e75e5b79c7bc51e1.

The pull request for this change was rejected, stating this is the
incorrect way to accomplish this. I was not able to find any
evidence that we still require this patch, so I am reverting it.
Adds an escapeForCSS method to SC.String.

The '.' and ':' characters are valid in HTML ids, but cannot be used in
a jQuery selector without being escaped. I added a method to escape a
string for use as a jQuery selector.
Updates jQuery framework to 1.8.3 and removes buffered jQuery.

Buffered jQuery theoretically improves performance by grouping all DOM manipulations through jQuery into a queue that can be flushed at once, but this needs to be benchmarked to be properly understood.
Patches jQuery to prevent security exceptions in IE.

This does NOT patch jQuery to make all calls to .append(), .insert(), etc. pass security validation in IE.  Instead it uses toStaticHTML (as recommended by Microsoft) to strip 3 innerHTML strings used within jQuery for testing.
Changes the patch of jQuery to use execUnsafeLocalFunction, which doesn't alter the test HTML used by jQuery and therefore should not effect the outcome of the jQuery tests while still removing the annoying IE warnings.
Fixes improper use of execUnsafeLocalFunction polyfill so that jQuery tests work properly.
Fix issue with null CSS values.

With the removal of buffered jQuery, null values used to clear layout
options are now being passed directly to jQuery. jQuery will ignore null
values for CSS properties, so we need to convert them to the empty string.

[obsoleted-by-master]
Bumping to SC 1.9.0.22.