Server Socket from File Desriptor

Update thumbor server to take a new option, -f/--fd, which expects a
file descriptor to create a socket from. This socket is then given to
the HTTPServer. This change makes it possible to run thumbor under
something like circus.
--fd -> --filedescriptor

Rename long option name and update variable names in code to reflect
this per pull #220.
Merge branch 'bind_file_descriptor' of https://github.com/198d/thumbor
Some minor changes on the work of @198d
removed filedescriptor parameter from context