file/send - Sends a new file.

Method parameters:
receiver_username - The Zvini username of the receiver.
name - The name of the file.
file - The content file to upload.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
ENTER_RECEIVER_USERNAME - The receiver username is empty.
INVALID_RECEIVER_USERNAME - The receiver username is invalid.
RECEIVER_NOT_FOUND - No such receiver with the username.
RECEIVER_NOT_RECEIVING - The receiver hasn't opened a connection to receive files from you.
ENTER_NAME - The name is empty.
SELECT_FILE - The content file is empty.
FILE_ERROR - An error occured while uploading the file.