doc:move plugin API documents from plugin.h to Sphinx text

TODO:

More describe it in doc/source/reference/api/plugin.rst
Translate it in doc/locale/ja/LC_MESSAGES/reference.po
Translate it in doc/locale/en/LC_MESSAGES/reference.po
Merge pull request #179 from naoa/doc-move-plugin-api-reference

doc:move plugin API documents from plugin.h to Sphinx text

Patch by Naoya Murakami. Thanks!!!