DOC/API: document how to use metadata
from [SO](http://stackoverflow.com/questions/26421221/propogation-of-metadata-from-dataframe-to-series/26423596#26423596)
xref #2485 
xref #7868
- [ ] documentation (maybe in the cookbook) to start.
- [ ] add _metadata to Index
- [ ] top-level metadata control (maybe)

```
def unit_meta(self, other, method=None):
     # combine and return a new unit for this

      if self.unit == other.unit:
          return self.unit

      # coerce unit
      return coerced_unit

pd.set_metadata({ 'unit' : unit_meta })
```

This last will require a bit of change in `__finalize__` to handle a metadata finalizer for a specific name (but straightforward)
