pil_plugin can import and export PIL image objects from memory

This pull request adds support for exporting and importing scikit images directly to and from PIL Image objects in memory, allowing users to do their transforms without writing the image to disk.

Two tests are added, following the file read/save tests. Some code was reorganized to prevent duplication.
I'd prefer this to read `if fp is not None:`. Implicit casts are not good for readability or maintainability.