[BEAM-1081] Annotations custom message support and classes tests.

[jglezt]

Added support for custom messages using the arg "custom_message".
The annotation decorator already supports tagging classes, I only added a few tests to check it out.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

[pabloem]

Hi Javier. Thanks for your contribution. There seems to have been a file that wasnt merged properly. Can you fix that? You can run tox from within sdks/python to run all of the unit tests, and lint checks

[jglezt]

Hi Pablo!
I resolved the merge problems.
Waiting for the Jenkins build.

[aaltay]

@jglezt it looks like there are test issues, could you look at those?

[jglezt]

Test issues fixed.

[pabloem]

Very cool. Thanks for the contribution again!