Embed branch titles in awkward docstrings for lazyarrays

[nsmith-]

Although python has nothing to say about instance docstrings, it seems IPython supports setting __doc__ on both class and class instances, in which case it displays both with the ? magic or shift+tab in jupyter. This PR takes advantage of that to display branch titles in lazy array docstrings.
This might not work in python 2, not sure. If not, I'll just use six to disable this feature for py2.

[jpivarski]

Chances are, it just doesn't do anything in Python 2. Old Python docstrings were more "hard-coded," and attempts to set them with __doc__ (at the class level) were just ignored. There was a hack to fix this by giving the class a custom metatype, which you can see in uproot.

Uproot doesn't depend on six, so if we had to set a switch, I'd rather just check sys.versioninfo (as you see elsewhere).

Is the PR done? I think lazyarrays gets checked in the tests, which passed (including Python 2).

[nsmith-]

The PR is done, feature-wise. It seems to work in my tests.