Allow specifying relative scales with fractional percentages

This pull request relaxes the layer-names parser to allow relative scales to be specified with fractional percentages like `33.33%`, and also adds on a few more unit tests to test these specifications. Note that the parser has *not* been relaxed to allow fractional percentages in the "quality" parameter. That is, `foo.jpg33.33%` still does not parse.

Addresses #174. 

CC: @gastonfig @nvasconcelos :gift: