rewrite demandload module

[Vadim Gelfer]

i have put change in crew that rewrites demandload module.

original demandload module only let you import simple python modules:

  import foo, bar -> demandload(globals(), "foo bar")

new demandload module is nicer. it lets you use demandload to replace
these python statements now:

  import foo.bar.baz  ->  demandload(globals(), "foo.bar.baz")
  from foo.bar import quux, zot  ->  demandload(globals(), "foo.bar:quux,zot")

speed is same as before, tested hundreds of times on commands that
should be fast. documentation is much better (old demandload had no
documentation at all).