don't alias the whole of OroGen to Orocos

This wrecks havoc with orocos.rb
loaders: allow to register callbacks when new projects/typekits get loaded
loaders: imported_typekits_for is supposed to raise when the type is not imported, do so
loaders: use normal typekit/project registration methods in RTT.setup_loaders
move some code that orocos.rb needs out of Gen into OroGen proper
define OROGEN_LIB_DIR, needed by the codegen part
TEMP: callbacks
loaders: fix PkgConfig.typekit_for
loaders: set the RTT typekit as virtual
spec: Spec::TaskContext really needs a project, force giving a first argument
loaders: declare void, nil, string and friends in the orocos typekit

It is needed by users of the loader API to get proper type resolution
when accessing RTT tasks
loaders: define opaque_type_for
spec: resolve_type only has one argument now, fix
loaders: fix deployment_model_from_name
rationalize some exception definitions
fix TaskContext#implements?

class_name is no more for a very long time
loaders: allow to manually register a type on a loader
loaders: define m_type?
loaders: declare the bool type from the RTT typekit
reviewed and cleaned logger access