Uproot

Fixed issue 16. The decompressor should be chosen based on the first two bytes of the compressed block, not the `fCompress` variable in TFile and/or TBranch.

For a while now, some uproot functions have been accepting a dictionary-like object as a cache, so if branches are in the supplied dictionary, they will not be extracted again from the ROOT file.

This version adds MemoryCache, ThreadSafeMemoryCache, and DiskCache (which is persistent and process-safe) as dictionary-like objects that can be used as caches. They all track memory usage (`numbytes` attribute) and evict the least-recently used items when it exceeds a threshold (`limitbytes`).

This code was mostly ported from the old PLUR project, adding process-safety to DiskCache so that a collection of servlets supervised by WSGI can share the same SSD cache (for example).

Bug-fixes during the preparation for LPC Computing talk. (Mostly bugs in the Arrowed connector, not main features.)

Added Pandas connector.

Added:

* Low-level baskets interface for truly zero-copy operations from the memory-mapped file.
* Interface to cast ROOT files as Arrow data structures through Arrowed (not currently available, so this feature will only get unlocked when Arrowed is released).
* Various bug-fixes.
* License at the top of all files is consistent with BSD-3 license for the whole project.

Pushed to PyPI.

Fixed vkuznet's issues (4 and 5) and pushed to PyPI.