Add the ability to upload files in a folder as events when they are generated

Sometimes you have a program that is periodically saving files containing data and you would like to have those files automatically uploaded as raw events to iotile.cloud. So, it would be great to have a program you could invoke and point at the folder containing those files and have it periodically check for new ones and upload them (optionally deleting ones that it has already uploaded).

There are two ways you could use it:
- a basic invocation accessible using `iotile cloud watch_folder` that will upload files that match a specific pattern as they are created.
- a more detailed class that can be customized in terms of how to summarize the raw events and additional tweaks like not uploading the newest file if it is still being updated.