Add posting of media files

The Open311 GeoReport v2.1 draft specification allows for sending of
media files associated with the reports. This is commonly used for
attaching a photo to go along with the report.

The draft is already implemented by some cities including Bloomington
and Helsinki.
Merge pull request #9 from juyrjola/master

Add posting of media files 
Update tests with files keyword
Merge pull request #8 from Jaakko/master

Adding jurisdiction_id into POST Service Request message if provided
Update version with Helsinki additions
Subtle requests bug