Pass headers to knox-mpu and auto detect content type if not set

This is a prop to add support to automatically set the `Content-Type` and pass `headers` to `knox-mpu`

Related to issue #1 and http://stackoverflow.com/questions/25206528/how-to-add-acl-and-content-type-parameters-when-using-skipper-s3

Checked in knox-mpu and seems that there is no need for changes. 

The `mime` package is also used with `knox` but only in a call to `putFile`.
https://github.com/LearnBoost/knox/blob/master/lib/client.js#L363