Add TableCaptionNode

Because we have a node for <table>, we also need one for <caption>,
otherwise we'll try to alienate it and fail.

Added the test case as a separate example document so Ed can use it
for his tests.

Removed test case asserting <caption> is alienated.

Change-Id: I3a917db58e6c0eb97899b214b07d01fc8d86b56d
Merge "Add TableCaptionNode"