mruby: enable "pp"
Add grn_operator_to_string() for debug
Add grn_p_scan_info_list() internal function for debug
Indent
Add missing include
windows: suppress a warning

     mrb.c: In function 'grn_mrb_get_system_ruby_scripts_dir':
     mrb.c:41:11: warning: unused variable 'path' [-Wunused-variable]
          char *path;
                ^
mrb: add missing "break"s
Add grn_inspect_indented() utility function for debug
Export grn_expr_get_vars() internally
Make grn_expr inspection more descriptive

The old implementation still exists.
Don't output ii values in grn_inspect()

Add grn_p_ii_values() for inspecting ii values.
Use include/groonga.h for meta header file

We can split contents in groonga.h to several files.
test: follow inspection result change
Export grn_inspect() and grn_p() family
select: support sequential match (no index match) with match_columns

Limitations:

  * Column must be one. "title || body" isn't supported.
  * Column must be value column. Index column isn't supported.

[groonga-dev,02902]

Reported by Atsushi Shinoda.
in_values: support auto sequential search mode

It will be faster when almost records are matched with index search.

It is used when

    (n_matched_records / n_index_searchable_records) > 0.01

"0.01" can be customizable by `GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO`
environment variable.

For example, `GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO=1.0` means that
sequential search is used rather than index search when the number of
matched records is less than or equal to the number of index searchable
records.

You can disable sequential search by specifying negative value to
`GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO` like:

    GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO=-1

Limitations:

  * Support only reference type column. It means that sequential search
    isn't used for `ShortText` type column.
  * Support only single column index.
  * Support only index that doesn't use weight.
  * Support only AND logical operator.
Add missing atomic operations which supports GCC atomic builtins

By this change, Groonga works on armhf platform now!

Note that it needs GCC 4.8 to build. GCC 4.6/4.7/4.9 may fails.
travis: try to use OS X

References:

  * http://docs.travis-ci.com/user/multi-os/
  * http://docs.travis-ci.com/user/osx-ci-environment/
travis: fix a typo

OS X can't be used for now.

http://docs.travis-ci.com/user/multi-os/ says that

> Consequently, we are not accepting requests for the multi-OS feature
> at this time.
travis: use 2 spaces for indent