CLI: send 'Accept:' header to ask for text/plain tracebacks. Closes #646.

CLI: send 'Accept:' header to ask for text/plain tracebacks. Closes #646.

The webapi has been looking for an Accept header since 1.4.0, but it treats a
missing header as equal to */* (to honor RFC2616). This change finally
modifies our CLI tools to ask for "text/plain, application/octet-stream",
which seems roughly correct (we either want a plain-text traceback or error
message, or an uninterpreted chunk of binary data to save to disk). Some day
we'll figure out how JSON fits into this scheme.