Add optional command-line parameter ('-f') to read dropbox config information (APPKEY, APPSECRET, etc) from a specific file allowing the script to support multiple dropbox destinations.

New usage looks like this for example: ./dropbox_uploader.sh -f myDBConfig upload /path/to/file
Merge pull request #45 from pjv/optional-config-file

Allow multiple config files for different dropboxes