`when` has an implicit `do`
Use built-in `subs` instead of `.substring`
Adds `substitute` fn to avoid nested `replace`
Indentation
Move 9-patch deletion outside of `resize-image`.

I'm not sure what 9-patches are, but I like the idea of `resize-image`
doing only what its name suggests :).
Use the `ns` import form
Merge pull request #3 from zachallaun/refactor

Code review!