PluginManager: use references instead of pointers where possible.
PluginManager: cleanup includes.
PluginManager: simplified and generalized PluginMetadata.

 * Removed all obsolete metadata members (name, authors, description).
   They were added back in 2007 when the purpose of this library was
   completely different (user-oriented desktop applications instead of
   developer-oriented high-performance middleware) and they can be
   easily implemented on top of current metadata, as shown in tests.
 * Reduced amount of data contained in each plugin. The plugin name and
   configuration pointer is common for all instances of given plugin,
   thus they were moved into metadata.
 * Added PluginMetadata::data(), which can be used to access additional
   plugin-specific data (i.e. adding information about authors etc).
examples: updated Plugin Management example.
Utility: removed Translator class.

It was created back in 2007 for just one purpose (translating plugin
metadata), but that is gone now, so this class is obsolete too. Moreover
the implementation was not exactly following good C++(11) principles and
should be reworked from scratch anyway.
PluginManager: doc++
PluginManager: cleaned up test.