Add height to Thumbnail
Add Thumbnail#aspect_ratio
Add Thumbnail#same_aspect_ratio?
Thumbnail#label returns nil when no label is present

A blank label can be forced with 300x_ or 300x400__2x, for example.
Rename same_aspect_ratio? to same_aspect_ratio_with?
Remove obsolete tests
Consider aspect ratios and labels when selecting thumbs

First, aspect ratios were not considered before. Second, labels were
considered in a non-flexible way. Now, if the requested thumb contains
no forced label, then label checking will be turned off.
Describe the thumbnail selection algorithm in the readme
v0.4.0