JsonReport improvements

[x] Will #getSeverity always return a valid Code Climate issue severity?
[x] Readup HTML vs Markdown => moved to its own issue #29
[x] What does #getExtendedDescription contain? Do we still need this comment?
- Empty string: no need.
[x] Add location when there no info about start/end lines VS. warning?
[x] Hardcoded base dir

codeclimate / codeclimate-sonar-java