Added support for attaching files

Features:

Added ability to attach files to the same thread as the message:

setting	required	description
files	no	Specify file or pattern of files to be uploaded to the same thread as the message posted.
max_files	no	Defaults to 5. This is to ensure the flow is not flooded if the file pattern matches too many files.

Breaking Changes:

Removed support for message type:

setting	required	description
message_type	no	Specify Flowdock MessageType. Currently supported types are: message, activity. If not provided, will default to message.