More relevant tests for `determineAppropriateResolution`

- definitely sorting out the relationship with image size and `data-width`
- expanding image based on container's size
Fixed interpolation.
`{width}` calculation is now performed against parent size.
Taking in account `data-width` as stated in the README.

Thanks Past Me for documenting it!
Merge pull request #86 from BBC-News/fix-86

Containers above max image size load smallest image