Stop exporting __all__; stop importing whichcell

Don't import anything from afs._fs into afs.fs; instead provide
a higher-level whichcell which returns None when the path is not
in AFS (rather than throwing OSError).  Rename the calls to
_fs functions as appropriate.
Add examine and whereis high-level functions

Add high-level functions for "fs examine" and "fs whereis".
Add a convenience function for DNS lookups.
Add logging to afs.fs
Use sizeof() in pioctl_write()

Don't assume that outbuffer is AFS_PIOCTL_MAXSIZE, just call
sizeof() and let the lower layer deal.
Drop support for python < 2.6

Require Python 2.6 in setup.py and remove the last vestiges of 2.5
exception syntax.
Release a new upstream version

Be bold and release version 0.2.0