added an optional headers param to Raca::Container#upload

* Use it to manually specify a content type, content disposition, etc
whitespace
don't assume the "file" binary is available on all systems

* if the user hasn't provided a content-type, choose a few common ones
  by file extension or just use application/octet-stream
added a details option to Raca::Container#list

* returns extra detail for each object - md5, size, content type, etc