set libxml error handler when a libxml-ruby error handler is set

This is a fairly small change to allow for ruby callers to set error
handlers for non-main threads.

Unfortunately, new threads will not inherit the error handler from
parent (or main) threads, but the code required to do this would
probably be fairly complicated and getting it working for both <1.9 and
>1.9 rubies sounds painful.

At least this way, provided users call XML::Error.set_handler close to
the call-site where they're using it, it's likely to do what they want
:)
Updated vc project file.
Merge branch 'bugfix/thread_local_error_handler' of github.com:geoffwa/libxml-ruby into geoffwa-bugfix/thread_local_error_handler

Conflicts:
	test/tc_parser.rb
Merge branch 'geoffwa-bugfix/thread_local_error_handler'
Merge branch 'master' of github.com:xml4r/libxml-ruby