grid api

that seems reasonable. how would you enter column names?

```
from plotly.grid_objs import Matrix

mat = Matrix(z, column_names = ['a', 'b', 'c'])
grid = Grid(mat)
```

or just:

```
grid = Grid(z, column_names = ['a', 'b', 'c'])
```

That's a bit confusing, cuz then you might want to be able to do:

```
grid = Grid([1,2,3], ['a', 'b', 'c'], 'column 1', 'column 2')
```

and how the column names link with the column data is implicit, which I don't really like.