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!!!
Follow path change in error message
Invert subrec size and GRN_RSET_SCORES_SIZE

Because score is stored before subrec:

    |-score1-|-subrec1-|-score2-|-subrec2-|...|
windows: maybe support Visual C++ 2010

[groonga-dev,02454]

Reported by cosmo0920. Thanks!!!
Add missing Visual C++ check
groonga-suggest-learner: add new options
doc: remove a resolved limitation for weight vector
Add 4.0.3 entry
doc: Groonga's HTTP server supports POST (experimental)
select adjuster: add an error check to avoid crash
Show the current error location by `|...|` on syntax error

For example:

    tags @ \"groonga\" * 4 |t|ags @ \"mroonga\" * 3
                           ^^^
Add more news item
Remove needless hg report
Show detected sphinx-build
doc: remove needless markups from cmdoption
doc: suppress a warning

    doc/source/troubleshooting/mmap_cannot_allocate_memory.rst:41: WARNING: Inline emphasis start-string without end-string.
doc: fix links
doc: translate
doc: fix links