Load CommandHelper lazily.

This one line for me on my machine consitutes a whopping **50** percent
of the `dzil nop` time in an empty directory.

Without this line commented out:

  strace -e trace=open dzil nop |& wc -l
  668

  time for i in 1 2 3 4 5; do dzil nop; done
  real   0m9.671s
  user   0m9.068s
  sys    0m0.596s
  cpu    99.94%

With this line commented out and done lazily when needed:

  strace -e trace=open dzil nop |& wc -l
  473

  time for i in 1 2 3 4 5; do dzil nop; done
  real   0m5.263s
  user   0m4.848s
  sys    0m0.412s
  cpu    99.95%

The only downside of this patch is there's a growth of this new warning
which I don't grok because I have no idea where its comming from:

  (in cleanup) Can't use an undefined value as an ARRAY reference at /home/kent/perl5/perlbrew/perls/blead/lib/5.21.3/autodie/Scope/GuardStack.pm line 47 during global destruction.

Fortunately, that is reasonably straight forward to fix and get even
more performance from by removing `use sanity`

  strace -e trace=open dzil nop  |& wc -l
  446

  time for i in 1 2 3 4 5; do dzil nop; done
  real   0m4.096s
  user   0m3.765s
  sys    0m0.328s
  cpu    99.95%

Yeah. That 'sanity' consitutes a sizable portion of startup cost too.
Merge pull request #24 from kentfredric/master

Load CommandHelper lazily.