Updated versioning and packaging

Version is now defined in bucket3.__version__
and used throughout the package and tools.

Important: If you have written your own scripts,
you should use "from bucket3.bucket import Bucket3" now.

Some minor modifications in README.rst
"reactions" are now "mentions".

I changed the nameing to match the idea of webmentions.
No functionallity was added or changed, just the naming of modules,
objects, command line options, etc, from "reactions" to
"mentions".