Combine drb_command_line and drb_options into one file.

These classes are related and will either be used
together or not at all. Seems better to just have
one drb file.
Add doc comment to get back to 100%.

Not sure why this is suddenly needed...
Merge pull request #1438 from rspec/drb-reorg

Combine drb_command_line and drb_options into one file.