Release 2.0.0 beta 4

Changes from 2.0.0-beta.3 to 2.0.0-beta.4

• New pluggable threading backend. Instead of having Blosc use its own
  thread pool, you can instead call
  blosc_set_threads_callback(threads_callback, callback_data) to install
  your own threading backend. This gives Blosc the possibility to use the
  same threading mechanism as one you are using in the rest of your program
  (e.g. OpenMP or Intel TBB), sharing the same threads, rather than starting
  its own threads that compete with yours for the CPU cores. See PR #81.
  Thanks to Steven G. Johnson.

• The endianness of the platform that is writing the data in chunks is stored
  now in the headers of the chunks. This info is not used yet, but this
  should allow a good hint for implementing format compatibility among
  platforms with different endianness in other layers. See PR #84.

• Fixed a nasty bug that prevented frames to go more than 2 GB in size.

• Added a cache for on-disk offsets. This accelerates the reading of slices
  from disk quite a lot (up to 50% with my benchmarks).

• Zstd codec upgraded from 1.4.0 to 1.4.3.