annotations should support custom messages and classes

[kennknowles]

Update https://github.com/apache/incubator-beam/blob/python-sdk/sdks/python/apache_beam/utils/annotations.py to add 2 new features:

1. ability to customize message
2. ability to tag classes (not only functions)

Imported from Jira BEAM-1081. Original Jira may contain additional context.
Reported by: altay.