Issues from PR#151 -- DTN integration

[spficklin]

Fixes were made to allow BDSS to communicate with a DTN to transfer files. That code was merged through PR #151. However, @nawatts raised some important points that need to be addressed. See the notes on the code within the PR comments for more details. Here's a checklist of items that need addressing:

• The temporary file code needs to be swapped out for something that doesn't create a local file.
• The DTN function needs to support --dry-run and --transfer-report options
• Check if URLs need quoting when sending to the subprocess function.
• Add new DTN config options to the proper README.