create meta type for `option`

Odo doesn't currently have a way of working with missing data.

For example, `pd.NaT` is not a `pd.Timestamp`; however, it is used to show 'no data'. We need a way to express this 'missing value' so that I can express something like `odo(None, option(int)) is None but odo(5, option(int)) == 5`

This would help blaze in situations like:

``` python
>>> d = bz.Data(None, dshape='?int')
>>> d
...
NetworkXNoPath: node not reachable from
```

Start with the `odo` entry point and the `option(int)` behavior described in the issue. Define how missing values and regular integers should be represented and converted, then verify that `odo(None, option(int))` returns `None`, `odo(5, option(int))` returns `5`, and the Blaze `?int` example no longer raises the reported error.

Written by the indexing model from the issue text.