Allow setting file content type based on file content(using libmagic)

[snoopie]

Currently the only way to set content type in carrierwave is using file extension(CarrierWave::MimeTypes).
There is a ruby-filemagic gem which offers ruby bindings to the libmagic library.
This commit adds libmagic-based processor which allows to detect file MIME type based on file content.

[bensie]

This is a good candidate for an external gem. We're not looking to add any additional processors to core given their dependency on external libraries that are not necessarily available. Looks good though!