initialize Port#model with an empty model if none is available
make NameService#each_task return an enum if no block is given
ruby_tasks: set a model for created permanent ports
fix type used to create port models in #port

We need to use the orocos type name, not the typelib type name,
or we'll try creating ports of intermediate types instead of
opaque types.
mention in manifest.xml which gems are needed for the test suite
Merge pull request #18 from rock-core/minor_updates

Minor updates