dual licensed under lgpl3 and apache2
a bunch of improvements

I did few improvements inside my couchdb branch.

- more efficient io thread wakeup and it'll now spin a bit for work
  improving microbenchmark results a lot

- fast path (read hitting page cache) now avoids message send and
  cleanup from io thread.

- writes are full async now. write will not wait for it. Instead reads
  will delays itself when necessary for in-flight writes.

- close (main path) and truncate are async now
allow plain reads
declare R16 support
removed copied copyright header from snappy-erlang-nif