Consolidate common tags definition and auto-generate language bindings

[yurishkuro]

Pulled out of #61

I think long term it would be great to have a yaml file in the docs repo that defines each tag (name, description, value type), then symlink the docs repo into each language repo as a git submodule, and write simple code generators. That way we would achieve uniform descriptions of the tags, make sure that each std tag is defined in each langauge, etc. The actual html doc can also be generated from the same yaml file. All the generators can probably be written once using the same template framework.

[bhs]

+1 -- great idea.

[bhs]

(c.f. #92)

[bhs]

... continued at opentracing/specification#10