examples/scew_stream: don't free tree in tree_hook, keep a reference instead
parser: only skip spaces if there's nothing in the stack

Before we skipped spaces only if we didn't found the <xml ...>
preamble. Even it's not corret, this might not be always present.
test/check_parser: add new unit test to verify chunked xml streams
bump configure.ac version to 1.1.6 and update NEWS file