Release 20.0: Experimental support for "retry on failure"

Release 19.3: Easy to use SSH tunneling support

Release 19.2: Add a glob() method to contexts

Release 19.1: Add context.is_executable() shortcut

Release 19.0: Report command output on failure (backwards incompatible*)

Refer to the new `really_silent' property for details about how this is
backwards incompatible. I suspect this to bite less than 1% of use cases
and I want executor to have sane defaults.

Release 18.1: Enable runtime processing of stdin/stdout/stderr

Release 18.0: Improve consistency of error handling (backwards incomp…

…atible)

Release 17.1: Support for Python callbacks in context.cleanup()

Release 17.0: Command execution in chroot using chroot

Release 16.1: Give contexts some `lsb_release' shortcuts