Various changes imported from a private repo.

These change are imported from an offline repo and don't really
constitute a patch with a single purpose.  Changes are:

* Prepended 'vips_' to some of the vips benchmark filenames.
* Added more benchmarks, some broken.
* Added tags to benchmarks in all_benchmarks.rb.
* Updated .gitignore to ignore image files.
* Updated the README.

These changes may break the benchmarks.  I suggest you skip to the next one.