Add 'legend' Method to Retrieve Current Legends

I tried this with
```
from bokeh.plotting import *
output_file('test.html')
line([1,2,3],[2,3,2], legend='blah')
legend()[0].orientation='top_left'
show()
```
and it seemed to have the desired effect. I'm totally new to bokeh and not really sure if this is indeed correct, but I couldn't find a method to easily retrieve the legends of a plot.