Support weight in UVECTOR

If uvector->header.impl_flags has GRN_OBJ_WITH_WEIGHT flag, the
uvector has weight information.

If an uvector has weight information, it uses the following format:

    |grn_id1|weight1|grn_id2|weight2|...|
     uint    uint    uint    uint

Uvector without weight uses the following format:

    |grn_id1|grn_id2|...|
     uint    uint
plugin ruby:use grn_plugin_command_create API instead of the macro
suggest:use grn_plugin_command_create API instead of the macro
table:use grn_plugin_command_create API instead of the macro
doc:add grn_plugin_proc_get_var and grn_plugin_proc_get_var_by_offset

TODO:

Translate it in doc/locale/ja/LC_MESSAGES/reference.po
Translate it in doc/locale/en/LC_MESSAGES/reference.po
doc:add grn_plugin_expr_var_init and grn_plugin_command_create

TODO:

Translate it in doc/locale/ja/LC_MESSAGES/reference.po
Translate it in doc/locale/en/LC_MESSAGES/reference.po
Merge pull request #177 from naoa/use-command-create-api-instead-of-macro

Builtin plugins use grn_plugin_command_create API  instead of macro

Patch by Naoya Murakami. Thanks!!!
Merge pull request #178 from naoa/doc-plugin-api-reference

Add plugin API reference

Patch by Naoya Murakami. Thanks!!!
Merge pull request #176 from groonga/support-weight-uvector

Support weight in UVECTOR
doc: remove garbage ";"
doc: add a missing "."
doc: fix English
mrb: support backtrace