Add regexp splitter replacement for re2_splitter: fixes #471

~~This PR add `reg_splitter` that is oniguruma based splitter plugin replacement for re2_splitter. This plugin reuses test of re2_splitter. Oniguruma support already merged via #465.~~
~~(discussion): Is this plugin name OK?, or is it better to create regexp_splitter such as built-in regexp_matcher/filter?~~

(2013-10-23 updated) This PR add `regexp` builtin splitter that is unified re2_splitter plugin and newly added oniguruma based splitter. We decided to make re2_splitter builtin function becase newer jubatus requires regular-expression library so we don't have to provide as a plugin.