Downgrade DZI manifest to XML namespace 2008

Lets us omit `ServerFormat`.
Remove IO from `DeepZoomImage` class
Remove `progress` module reference
Remove comment for computation shortcut
Consolidate `getTileBounds` and `getTileURL` into `getTile`

Also: Add `url` to `level`.
Extract ImageMagick converter
Add benchmark
Add VIPS benchmark
Add VIPS support
Run test with VIPS
Extract defaults
Format manifest XML
Fix incorrect argument name
Rename main file
Auto-detect fastest image library

/cc @iangilman
0.0.2
Merge pull request #1 from openzoom/2-vips

VIPS support

Closes #2