turned common `tags_as` into static method

this makes it easier to be reused (e.g. by external tools)

note that this can still be used as an instance method:

    SerializationInterface.tags_as(tags) == SerializationInterface().tags_as(tags)
moved tags serialization into tiddler module

alongside the existing deserialization function
added/updated docstrings for tags (de)serialization
Merge pull request #68 from tiddlyweb/tags-serialization

turned common `tags_as` into static method