Fix an arithmetic overflow which can cause segv.
Merge.
travis: test mruby build
doc: describe about tuning parameters

TODO:
  * Update file list
  * Translate
Revert "Indent"

This reverts commit c7db6bf753b4f8e25757e2447e82469d7b4cf2cc.

Now, the indent is needless. See the next commit for details.
Revert "Add missing lock for table creation"

This reverts commit 1f1a77402e22a75ca74d488694cd584ac8566dc6.

The lock isn't needed because process global IO objects are guarded
by lock at grn_io_register() and grn_io_unregister() in io.c.

The lock is taken for temporary table that is used for search result.
So the lock may cause a performance regression bug. (We should run
a benchmark but we don't yet...)
Load "expr.rb" script from mrb_expr.c