Move dochandler out to separate module
Convert docstrings to comments

Not part of the public API.
Move subcommand table generation into s3 class itself

No need for this to be a handler.
Change casing of PARAMS_DICT

For pep8.
Rename S3Command to S3SubCommand

To be consistent with naming conventions in the CLI.
Add precondition check for s3 destination resources

There was already a check for the source resources, but not
for the destination sources.  Now, before actually trying to
run commands, we validate that the s3 resources exist.

Tests added that show the new error cases.  I kept the 255 RC,
as that's what clidriver is currently using for uncaught exceptions.

Fixes #376.
Merge branch 'non-zero-rc-s3' into develop

* non-zero-rc-s3:
  Add precondition check for s3 destination resources
  Rename S3Command to S3SubCommand
  Change casing of PARAMS_DICT
  Move subcommand table generation into s3 class itself
  Convert docstrings to comments
  Move dochandler out to separate module