Reintroduce jam rule for creating remote repository for testing.

* 'jam build-remote-test-repository' can now be used to create a
  remote repository which will then be used by the build system,
  such that the resulting package set can be tested.
Publish updated perl, python, git, mercurial and scons packages.

* perl and python now support vendor-folders for modules and use correct
  (i.e. writable) site-folders
* git, mercurial and scons have been adjusted to put their perl/python
  modules into respective vendor-modules folders
Update 'jam help' output.