Implement document fragment parsing.

Introduce a notable behavior change in default parsing mode: documents without a
document element node are now considered invalid. This is technically a breaking change,
however the amount of documents it affects is very small, all parsed data still persists,
and lack of this check results in very confusing behavior in a number of cases.

In order to be able to parse documents without an element node, a fragment parsing flag is
introduced.

Parsing a buffer in fragment mode treats the buffer as a fragment of a valid XML.
As a consequence, top-level PCDATA is added to the tree; additionally, there are no
restrictions on the number of nodes -- so documents without a document element are considered
valid.

Due to the way parsing works internally, load_buffer_inplace occasionally can not preserve
the document contents if it's parsed in a fragment mode. While unfortunate, this problem is
fundamental; since the use case is relatively obscure, hopefully documenting this shortcoming
will be enough.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@980 99668b35-9821-0410-8761-19e4c4f06640