Popup width

[ocefpaf]

@Conengmo this solves problems like #959 (comment)

[Conengmo]

Looks good @ocefpaf. One caveat though is that popups with only text become more narrow than they used to be. For example the 'Fancy HTML popup':

http://nbviewer.jupyter.org/github/ocefpaf/folium/blob/popup_width/examples/Popups.ipynb#Fancy-HTML-popup
http://nbviewer.jupyter.org/github/ocefpaf/folium/blob/master/examples/Popups.ipynb#Fancy-HTML-popup

A solution is that now users of text have to give in a value in pixels. But maybe there's a way to have it work automatically for both graphics and text?

[ocefpaf]

Looks good @ocefpaf. One caveat though is that popups with only text become more narrow than they used to be. For example the 'Fancy HTML popup'

Yep. In a way that is the "default" 100% width reserved for text from leaflet. We can always explicitly request more like in cell number [3].

A solution is that now users of text have to give in a value in pixels. But maybe there's a way to have it work automatically for both graphics and text?

We could try to add some logic to add that behavior. I understand that the behavior in this PR is "kind of a breakage" from the previous one, but now things are more closely related to how leaflet works and less "magic."

I'm not against checking for text vs other object to use two defaults, 300px and 100% respective, but I have to confess I'm not in love with the idea of the extra complexity there.