Uproot

Fixed 246: error messages pertaining to file formatting now print out which file has failed. Useful if you are running a large job over many files and want to find the one that broke the job.

Added support for reading `std::map<std::string, X>` where `X` is a numerical type or a `std::string`. (PR 245)

Requests for `array(...)` or `arrays(...)` through HTTP and XRootD now start asynchronous downloads of all the basket data before starting to read, decompress, and interpret. This keeps the network busy prefetching while the CPU is preoccupied, hiding latency. The HTTP preloader is implemented with `concurrent.futures.ThreadPoolExecutor` (not selected by default in Python 2, as that would require a non-standard library dependency), and the XRootD preloader is implemented with a pyxrootd callback. The `threads` parameter is a number of threads for HTTP and a boolean for XRootD: yes-parallelize or no-don't, because we don't control how many threads pyxrootd uses. (PR 242)

Binder now uses JupyterLab, rather than Jupyter Notebook. (PR 244)

Moved Event.root to an HTTP source to make the tests directory smaller.

`uproot.numentries` is now documented.

Deploy another one because the last already exists in PyPI (not a clean test).

Merge PR 235, which only adds to tests, but it required a removal of PyOpenSSL from Travis, so I need to know if it will deploy properly (the reason the PyOpenSSL update was in there).