unicorn 5.1.0 - rack is optional, again

Note: no code changes since 5.1.0.pre1 from January.^WNo, wait,
last minute performance improvement added today.  See below.

The big change is rack is not required (but still recommended).
Applications are expected to depend on rack on their own so they can
specify the version of rack they prefer without unicorn pulling
in a newer, potentially incompatible version.

unicorn will always attempt to work with multiple versions of rack
as practical.

The HTTP parser also switched to using the TypedData C-API for
extra type safety and memory usage accounting support in the
'objspace' extension.

Thanks to Adam Duke to bringing the rack change to our attention
and Aaron Patterson for helping with the matter.

Last minute change: we now support the new leftpad() syscall under
Linux for major performance and security improvement:

    http://mid.gmane.org/1459463613-32473-1-git-send-email-richard@nod.at

8^H9 changes since 5.0.1:

      http: TypedData C-API conversion
      various documentation updates
      doc: bump olddoc to ~> 1.2 for extra NNTP URL
      rack is optional at runtime, required for dev
      doc update for ClientShutdown exceptions class
      unicorn 5.1.0.pre1 - rack is optional, again
      doc: reference --keep-file-descriptors for "bundle exec"
      doc: further trimming to reduce noise
      use leftpad Linux syscall for speed!