Warn when image type conflicts with reported media type

```
Currently, flightcrew reports when a file extension disagrees with the reported
media-type in the OPF file. (error 1107, or ERROR_OPF_BAD_ITEM_MEDIA_TYPE_VALUE)

Sometimes, even the file name is wrong. I encountered an ePub file that had an
image.jpg with the media-type of 'image/jpeg' that was actually a PNG.

This prevented the Sony Reader from displaying the image.

The error was correctly caught by the latest EpubCheck. Flightcrew should have
the same test.
```

Original issue reported on code.google.com by `saltyho...@gmail.com` on 10 Mar 2012 at 10:42