Don't shade the DKIM dependency anymore

[bbottema]

Simple Java Mail is now fixed to specific versions of some minor external libraries. However, as the only 3rd party dependency, Markenwerk's dkim library (added for #33) should be depended upon the same as any other dependency rather than being shaded into the jar under a different package.

This solves both #80 and mronager@5b98c85

It might solve the problem for upgrading outdated dependencies. But there are still a few problems:

• the dkim libraries depend on other libs, which are not included in the shaded jar (at least i had issues with this)
• it is unclear which dependencies are pulled in, since they dont show in the list
• it's not possible to exclude dependencies when they are shaded (e.g the outlook lib)

[bbottema]

Issue done, I'm just waiting for a reply from @toKrause on whether the update from 1.1.6 to 1.1.10 might give us any surprises.

[bbottema]

Released as 5.0.0.rc1-SNAPSHOT. Add OSS' snapshots repo to find it in Maven.