retroshare command line interface

Tested with retroshare 0.6.5 and python 3.7. It should work with py 3.6 and maybe even with something older, who knows.
In case you test it with `python<=3.5`, write me a note.

## Installation

    pip install .

## Usage


    API_USER=user:pass  # as specified in the GUI
    rscli  --auth "$API_USER" channel list
    rscli  --auth "$API_USER" channel --channel-id abcdef123456 show
    rscli  --auth "$API_USER" channel --channel-id abcdef123456 read
    rscli  --auth "$API_USER" channel --channel-id abcdef123456 read --long

    rscli  --auth "$API_USER" forum list
    rscli  --auth "$API_USER" forum --forum-id abcdef123456 read
    rscli  --auth "$API_USER" forum --forum-id abcdef123456 read --long

    rscli  --auth "$API_USER" file publish --fname foo.txt