Version 0.6.1

Minor bugfix, performance and compatibility release.

• Minor compatibility fix: on Linux, drop the use of the attr library,
  and instead switch to the glibc header sys/xattr.h, which is
  provided for a really long time (since glibc 2.3). The formerly used
  header attr/xattr.h has been removed from the attr library in
  version 2.4.48. Fix provided by Lars Wendler, many thanks!
• Release the GIL when performing I/O. Patch proposed by xwhuang, many
  thanks. I tested this a long while back it seemed to impact
  performance on local filesystems, but upon further inspection, the
  downsides are minor (between 0 and 5%, in many cases negligible). For
  remote or slow filesystems, this should allow much increased
  parallelism.
• Fix symlink set operation on MacOS X; bugfix provided by adamlin, much
  appreciated! This also uncovered testing problems related to symlinks,
  which are now fixed (the bug would be caught by the updated tests).