Feature: file_download helper function

[chrisbc]

As a user I want the client to provide me a simple download helper so that I can fetch files as easily as possible from the ToshiAPI, and it

• should also let me rename the file
• should let me specify a path to save to

NB both these can be supported with similar logic to posix mv command .i.e

When you specify a single source file and the target is not a directory, mv moves the source to the new name, by a simple rename if possible.

Done when:

• the client has a file_download helper function
• uses the usual ENV variables by default
• supports saving to specific output folder
• supports renaming the file
• If user specifies a ToshiID that does not support FileInterface then throw a helpful exception.
• on success return the final name of the saved file.