Fix String#scan argument

* Add String. Not only Regexp.
* Fix description when block given(looks same as String#gsub)
Fix {Regxp,String}#match

* Adjust String#match API summary
* Write with block API for 1.9.1 and later
Merge pull request #35 from kachick/fix-string-scan,match

Fix String#scan and {Regexp,String}#match