save openslide metadata to .dzi files

see https://github.com/jcupitt/libvips/issues/137

todo:

- save to zoomify ImageProperties.xml as well?
- any other useful fields?
write props to a separate file

and dump all vips props
wrap props in another element
note date and version in dzprops
fix a bash-ism in configure.ac

helps freebsd
Merge branch 'master' into dzsave-metadata

Conflicts:
	libvips/foreign/dzsave.c
more tweaks to dzsave props

add docs too
Merge branch 'dzsave-metadata' of github.com:jcupitt/libvips into dzsave-metadata

Conflicts:
	ChangeLog
	configure.ac
more dzsave props tweaks

* don't version xmlns
* escape <> in field names and values
* set a type on each value
use libxml2 to write props

don't write xml ourselves, get libxml2 to do the encoding
sync
pretty-print dzsave xml output
put type on the value, not the field name
oops
Merge branch 'master' of github.com:jcupitt/libvips

Conflicts:
	TODO
Merge branch 'dzsave-metadata'

Conflicts:
	ChangeLog