* Added the set_chunk_size handler call in the dealer
  to allow drivers to force a specific chunk size (restricted
  by some services)
* Added the documentation for set_chunk_size handler
Merge pull request #40 from onitu/#38_set_chunk_size

Added the set_chunk_size handler call in the dealer...
Amazon S3 Driver : Proof of Concept
* Rollback to the real Proof of Concept original version instead of the broken multipart one
* Modification to pass flake8 build
* New version of S3 driver : added manifest.json

* Making the changes to match the new way to handle the Plug

* Added some exception catches and documentation
* Refactoring of the start function

* Added the get_conn function
* Moved some error handling code.
  It didn't try to catch exceptions at the right place.

* The get_bucket function now takes a boolean arg, allowing
  to ask it to only check the bucket's existence without trying
  to fetch contents (it is faster).

* New get_file function that raises ServiceError if file is not found
  on S3. Better than to try and raise in the several functions that do it

* Now handling some S3 read timeout errors by raising TryAgain exceptions.

* Encountered a socket.error exception raised by boto in the check changes thread
  when connection reset by peer.  Handling it by trying a reconnection

* Moving from concatenated strings with '+' to str.format()
* Fix flake8
Return of the multipart upload approach.

* Get rid of the global variables as it wasn't thread-safe

* Add helper functions get_file_timestamp and get_file

* Add the set_chunk_size handler, as S3 requires parts of at least 5 MB

* Add some documentation, or modify existing one

* The bucket polling thread now checks for and skips the files that are
  currently being uploaded. This used to cause transfer problems.