Added functionality for type = 'files'

# Added File Push Capability

* Added a function `.getUploadRequest()`to perform an upload request to the Pushbullet API.
* Added code to `pbPost()` in *pushes.R* to allow the option `type='file'` to upload the file, and then send the file. *Very limited error checking is included at this time.*

## Notes
* Noticed that when `device_iden` was blank in the curl request, the API would return a server error. There is an `ifelse` statement included to remove the `device_iden=` when `tgt` is blank.
* This is my first pull request; hopefully I've done this correctly & code style isn't too atrocious!