First go at formats
Use struct instead of class

Format does not need to have state.
Merge conflict
Format unit tests
Merge branch 'master' of https://github.com/hpique/HanekeSwift into feature/formats/intro
String extension to escape filenames
Escape key when used as filename
Merge pull request #36 from hpique/feature/formats/intro

Introduce formats. Closes #26.
Fix merge conflic
Merge pull request #37 from hpique/feature/disk-cache/escape-key

Escape invalid key characters for disk image filename. Closes #16.
Fix merge conflicts