use ETag and Last-Modified headers in Middlecoin JSON requests

    send headers with middlecoin requests to let minimally smart webservers sent us 304 that tells us the data has not been modified.
    - reduces downloaded data to just updated JSON, not every request
    - reduce request time to 5 minutes
    - minor output formating change
Merge pull request #2 from nichenke/master

 use ETag and Last-Modified headers in Middlecoin JSON requests