Add logger to BokehJS

This PR addressed #722 by adding [JSNlog](http://js.jsnlog.com) which is a simple small JS logging library.  It has an AJAX appender that we can explore later for sending log information to the Bokeh Server for easy inspection. For now it is configured with a console appender that looks like this:

![screen shot 2014-08-24 at 12 33 00 pm](https://cloud.githubusercontent.com/assets/1078448/4024052/1c683188-2bb5-11e4-8c60-af488088d666.png)

Some tasks remain:

* [x] ~~need unique logger per... plot? document?~~
* [x] find good mechanism to configure log level from python
* [x] add some log statements to trace general program flow at a high level

@mattpap, @hhuuggoo input on the second question is appreciated.  Some kind of global variable that gets passed on `window`? It would be good for this information to be immediately available so logging can start as early as possible (i.e., not be part of some model, I think, otherwise we would have to wait for all the models to be loaded before logging could be used).