fedora: ensure that log directory exists on startup
doc: write in English about Users table
doc en: translate into English about Comments table
doc en: translate into English about HashTags table
doc en: translate into English about bigram table
doc en: update example data about loading data section

TODO: improve more suitable examples for foreigner
doc en: translate into English about loading data description
doc en: add missing space

Without this change, it doesn't regarded as part of groonga commands.
doc en: update query in English
doc en: update tutorial examples
mrb ctx: cache singleton object
mrb: define error classes

anzu-query-replace-regexp is my friend. :-)
mrb expr: bind grn_expr_alloc_const()
mrb expr: bind grn_expr_take_object()
mrb: support optimize for "X > 0 && X < 10"

    X > 0 && X < 10
    ->
    between(X, 0, "exclude", 10, "exclude")

    X >= 0 && X < 10
    ->
    between(X, 0, "include", 10, "exclude")

    X > 0 && X <= 10
    ->
    between(X, 0, "exclude", 10, "include")

    X >= 0 && X <= 10
    ->
    between(X, 0, "include", 10, "include")
mruby: fix a bug that flags isn't copied
mruby: fixed a bug next_data isn't removed
mrb mrb_ctx.h: If it disables --enable-mruby, mrb_state type doesn't exist in this scope.

It generates following error:
```log
CC       ctx_impl_mrb.lo
In file included from ctx_impl_mrb.c:26:
./mrb/mrb_ctx.h:29:24: error: unknown type name 'mrb_state'
void grn_mrb_ctx_check(mrb_state *mrb);
                         ^
1 error generated.
make[4]: *** [ctx_impl_mrb.lo] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
```
Merge pull request #200 from cosmo0920/mrb-ctx-ifdef-block

mrb mrb_ctx.h: fix error: unknown type name 'mrb_state'

Patch by cosmo0920. Thanks!!!
benchmark: add benchmark for range search

It can be optimized by mruby.