examples: Update hello example with recent changes.

Update the "Hello World!" plugin example to match
the recent changes that we've made in the plugin structure.
docs: Update documentation about loading external plugins.

Make it clear that we need to prefix every external plugin
with "avocado_" to load it. Also, using the new updated "Hello
World!" plugin example.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #221 from ruda/master

Update documentation regarding external plugins and the hello world plugin example.