Add a parameter checksumContent to generate a custom checksum

Default on the message if the checksumContent isn't provided
Use a checksumContent in order to group exceptions and messages

Exceptions are now grouped by stacktrace content which should be more
relevant than exception messages
Simple logs are grouped by locationInformation (line where the log was done)
if available, message otherwise.
Fixed code samples
Merge pull request #39 from ianwalter/master

Fixed code samples in README.md
Update README.md

Make sure to actually use the logback appender in your config.
Merge branch 'grouping' into oxford
Set a custom version for raven-oxford
Add oucs maven repository as the distribution management