make select much more useful

Can you turn `tag` into a property or a setter, depending whether we want this be send over the wire? I would be an unhappy user to learn that we take advantage of this:
```
In [1]: from bokeh.objects import *

In [2]: p = Plot()

In [3]: p.tag
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-e69029922ae7> in <module>()
----> 1 p.tag

AttributeError: 'Plot' object has no attribute 'tag'

In [4]: p.tag = ["a"]

In [5]: p.tag
Out[5]: ['a']
```
I thought this was already banned, but apparently not.