Give name to global rednotebook logger object.
Macro plugin infrastructure implemented.

With this commit, rednotebook now has a simple macro plugin
infrastructure. By adding a {PLUGIN:plugin_name} tag to a template,
plugin_name's get_text() method will automatically be called to replace
the tag whenever the template is inserted.

plugins.py - contains the RedNotebookPlugin superclass and logic for
    loading/finding plugins.

rnplugins/wikipedia_anniversaries.py - example plugin that grabs the
    selected anniversaries (today in history) from wikipedia.

templates.py - _replace_macros() function modified to support plugin
    interface.