Improved the Raven handler when using a Buffer handler
Amended PR #203 (RavenHandler batch improvements)
Filtered records based on their level
Took the record with the highest severity as the main one
Merge pull request #1 from pvanliefland/raven-improvements

Amended PR #203 (RavenHandler batch improvements)
Merge remote-tracking branch 'fabpot/raven-improvements'
Fix issue in handleBatch when all records are below the minimum level, refs #203
Rename logFormatter to batchFormatter, refs #203