Fix open function

`Viewer.open()`: fix docs, tileHost

* Updated documentation to match actual parameters (see #53)
* tileHost was completely broken for dzi files. Now it's based on the DZI source URL (e.g. `foo.dzi` -> `tileHost` + `foo_files/` vs. `_files/`).
* `processDZI`: was broken in c41f6a464e8c7676abd92f0b1a7d3ea80b337d12 when `imageFormatSupported()` was turned into a method