Added support for setting custom headers for files uploaded to s3.
Updates issue #7 - Brings back S3_USE_CACHE_CONTROL setting and fixes
some documentation issues.
Updates issue #7 - Set old S3_CACHE_CONTROL in S3_HEADERS on init and
doc fixes.
Check for S3_USE_CACHE_CONTROL boolean
Merge pull request #7 from hannseman/master

Adds support for setting custom HTTP headers, on files uploaded to S3