DEV: Make callable used to create a Process configurable.

[ssanderson]

Allows users to specify an alternate callable to produce an object with the same interface as a multiprocessing.Process.

Most notably, this is useful for spawning a server with a subclass of multiprocessing.Process, such as the Gevent-aware Process subclass implemented by gipc.

[jcomo]

Looks good, thanks! Please add some documentation about this before we merge it in