Make sure that GetFileURL can correctly handle test environments bots

[PaulSonOfLars]

What

Change the GetURL method on the File object to ensure that it supports getting files from the test env. Addresses #101.

Impact

• Are your changes backwards compatible? No - method definition changes as well as new methods and method renames on the BotClient interface.
• Have you included documentation, or updated existing documentation? Y
• Do errors and log messages provide enough context? Y