Implemented 'bldsup' local plugin support.

Check out this implementation and let me know what you think. I couldn't use properties as I originally suggested since properties are set in `build.py`, after imports and `use_plugin()` calls.  Doh!

So I added a new `use_bldsup(build_support_dir="bldsup")` function in `pybuilder.core`. If it's not called in a project's `build.py`, then no "bldsup" directory is searched.  If it is called, then it confirms that the `build_support_dir` directory exists and that includes a `__init__.py` file so imports will work.  It just needs to be called before you try to import/`use_plugin()` and modules from the `build_support_dir`.

I also moved the `pdoc_generate` task into a "build specific plugin" in `bldsup/pdoc.py`. Mostly this is for illustrative purposes as.