update Action#toString to be more verbose RE: headers
replace hand-coded MIME fixing with “mime” NPM module
fix issue where only the initial 1000 files in S3 were compared
fix MIME detection for no-extension files like “README”, etc
fix (and test) MIME detection for certain edge cases
re-upload if headers are wrong (fixes #10)
bump version to v0.4.0
Merge branch 'release/v0.4.0' into develop