[WIP] ListCollection mpl-support 

Hi people... This is WIP and it is not ready to merge yet.
I have some problems I would like to let you know, to help me find a way to make it work.

The proposal is add the LineCollection support as a Multiline bokeh glyph. So, I have written a `_make_lines_collection` function which mimic `_make_line` function, but using a Multiline glyph instead of a Line one.
The matplotlib LineCollection support the `get_segments` method which gives you the `ends` of each segment, so after dealing with the list provided (containing arrays with coordinates), I could build the `xs` and `ys` lists to pass to the Multiline glyph...
So, I have also written an example of LineCollection to use it as a test case... But the resulting html is not showing the figure. When I see the JSON, the data is there, but the plot is not rendered...
And I don't have any error message to understand why is not working...
Maybe my test case is flaw... or I am missing something in the implementation...
Can you take a look?

Thanks.
PS: pinging @bryevdv, who discuss this with me a little bit by email... but anyone else is invited ;-)