• Added appengine build tag for Windows. When enabled, memory-mapping will be
  disabled in the Windows build as it is for the non-Windows build. Pull
  request #35 by Ingo Oeser.
• SetFinalizer is now used to unmap files if the user fails to close the
  reader. Using r.Close() is still recommended for most use cases.
• Previously, an unsafe conversion between []byte and string was used to
  avoid unnecessary allocations when decoding struct keys. The decoder now
  relies on a compiler optimization on string([]byte) map lookups to achieve
  this rather than using unsafe.