String extension to escape filenames
Escape key when used as filename
Make extensions internal
Use implicitly unwrapped optionals
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
Add diskCapacity to format
Make format diskCapacity immutable
Merge pull request #38 from hpique/fix/minor

Minor fixes: use implicitly unwrapped optionals and make all extensions internal
Merge pull request #40 from hpique/feature/formats/disk-capacity

Format disk capacity. Closes #29.
Merge branch 'master' into feature/formats/resizing