Add Histogram example and docs

[fonnesbeck]

The new Histogram class from #1904 is missing documentation describing its usage, as well as a working example. The latter can be either a simple model in the examples folder or a fully annotated notebook if it is merited.

[ferrine]

@fonnesbeck I can fix it on weekends

[ferrine]

Here will be a good example of using Histogram in the right way

[twiecki]

Let's close in favor of the PR.