Expose 'density' attribute for magickload

Allows control of vector format resolution
quieter gobject-introspection scan

and make sure gtk-doc.make isn't a symlink
maybe this fixes travis
Merge pull request #200 from lovell/magickload-density

Expose 'density' attribute for magickload
add notes to logs for @density
try to fix travis again
arg another attempt to fix travis
ignore missing class methods in Vips.py

thanks Ben

see https://github.com/jcupitt/libvips/issues/199
expand tiff write res range

see

https://github.com/jcupitt/libvips/issues/206
output ref_count in object summary

tiny Vips.py fix too
zero out edge tile buffers before tiff jpeg write

see https://github.com/jcupitt/libvips/issues/204

thanks iwbh15
add tiny demo prog
small cleanups
add python3 support
fix default interpolator in similarity

was not defaulting correctly
note