Make autodie wrappers conditional on the version of Perl.

Rather than removing functions completely from autodie, which will
break backwards compatibility with older perls, autodie will know
which functions have been implemented internally and skip the wrapper.

It also sets $^H{"autodie/$func"} = 1 to signal internally to use
autodie for that function.  is_autodie_enabled() has been changed to
look for that.  This allows autodie to be turned on and off per
function even if that function is implemented internally.

For #13 and #10
Implement autodie in chown().

Picked it because it has special error checking logic.
Make "no autodie" nested inside "use autodie" unset $^H{autodie/func}.

This is vital to the internal autodie implementation working.
CORE::blah will no longer work around autodie.

This will be a backwards compatability concern.  I don't think there's
anything to be done about it.
Add autodie to chown, kill, unlink, utime, chmod

These are all the IO functions which return the number of files
succeeded and thus need a little more logic to determine failure.