Rewrite 'makedist' to support Ruby 1.9
When building P4Ruby with 'jam', 'makedist' will fail because it
contains 1.8 specific modules and methods. This change separates
this out, moving non-version specific methods to the module
P4RubyCommon (makemodule.rb). Version specific stuff has been moved
to two separate classes; P4Ruby18 (make1.8.rb) and P4Ruby19
(make1.9.rb). 'makedist' now checks RUBY_VERSION and creates an
instance of the appropriate class depending on the version of Ruby
used.