Embed mruby to grn_ctx_impl

It just embed. We cannot run Ruby code yet. We need likethe following
code to run Ruby code:

  {
    mrb_state *mrb = ctx->impl->mrb;
    struct mrb_parser_state *state;
    int code_start_index;

    state = mrb_parse_string(mrb, "p Time.now", NULL);
    code_start_index = mrb_generate_code(mrb, state);
    mrb_pool_close(state->pool);
    mrb_run(mrb, mrb_proc_new(mrb, mrb->irep[code_start_index]), mrb_nil_value());
  }
mruby: remove needless Ruby check
mruby: add MRUBY_CFLAGS to build options
mruby: extract mruby related code to another file
mruby: support disable by GRN_MRB_ENABLED=no environment variable
Merge pull request #50 from groonga/mruby

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