Rename exception.h to errors.h.
Add classes for better errors handling.

Add xml::error_handler class that implements libxml2 error and warning
callbacks. Include some commonly needed specializations: error_messages
for collecting the messages in a log and classes for throwing on errors
or on errors and warnings. The latter two are to be used through
xml::throw_on_error and xml::throw_on_error_or_warning global instances.

The error_messages class is inspired by public domain code by Sergey
Satskiy <satskyse@ncbi.nlm.nih.gov> from National Center for
Biotechnology Information
(see http://anonsvn.ncbi.nlm.nih.gov/repos/v1/trunk/c++).
Add xml::schema class.
Add tests for xml::schema.
Allow customizing error_messages::print() formatting.

Add format_for_print() to allow customization of the prefix ("error: "
and "warning: " by default).
More efficient error_messages::has_{errors,warnings}.
Ignore admin/test-driver script.
Convenience ctor for throwing error_messages-based exception.
Use error_handler everywhere it makes sense.

Add appropriate constructors and overloads to tree_parser and
stylesheet.

Notice that bool-returning xslt::stylesheet::apply() methods cannot have
the usual default of xml::throw_on_error because of backward API
compatibility.
Don't use deprecated APIs in tests.
Add xml::document ctors for parsing.

Avoid the need to use tree_parser class, add convenience constructors to
create xml::document from data or from file.  This is consistent with
xml::schema and xslt::stylesheet classes.

Note that the document(filename) ctor cannot have default value of
on_error argument because of ambiguity with the existing
document(rootNodeName) ctor.  Which is why the latter is deprecated.
Don't use deprecated xml::document ctor in tests.
Support Clang's deprecated attributed with message.

Clang can include more specific message in __attribute__((deprecated))
to tell the programmer how to fix the warning.  Use it, it's quite
useful.

See http://clang.llvm.org/docs/LanguageExtensions.html#messages-on-deprecated-and-unavailable-attributes
Allow comparing const_iterator with iterator.

Fix the definition of xml::node's and xml::nodes_view's operator== and
operator!= to make comparison between const and non-const iterators
possible.

Add tests for this.
Update homepage links to point to GitHub Pages.

Also add a link to SourceForge.net downloads.
Update libtool versions before release.

libxmlwrapp's version was already increased on master, compared to
0.6.3.  But some signatures changed (while preserving API
compatibility), so reset age to 0.  libxsltwrapp got interface changes.
Increase version to 0.7.0.