Support for sending a POST request to custom (a plugin) REST resource

I have adapted py4dmx to be fit for my case.

Could you please review the code changes I will reference in this issue?

It would be great if we could integrate this feature somehow in the dmx.py core functionality.

As I didn't know how to integrate my functionality I added a new (cryptic) CLI arg named -SP, --send_post

I use the adapted script the following way and it works for me:

python dmx.py -SP "rdb-connector/persons" -w "My Custom Workspace"

Note: python3 gives me the following warning when I execute the script:

dmx.py:56: DeprecationWarning: The SafeConfigParser class has been renamed to ConfigParser in Python 3.2. This alias will be removed in future versions. Use ConfigParser directly instead.
  config = configparser.SafeConfigParser()

I am currently unaware if its possible to continue to support python2 after renaming SafeConfigParsertoConfigParser` dependency, or if this requires the script to be aware of its runtime environment (=> differ for python2 and python3).