unicorn 5.1.0.pre1 - rack is optional, again

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.

There might be more documentation-related changes before 5.1.0
final.  I am considering dropping pandoc from manpage generation
and relying on pod2man (from Perl) because it has a wider install
base.

5 changes since v5.0.1:

      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