Make str_modify public
Merge pull request #1188 from h2so5/make-str_modify-public

Make str_modify public
Add String#swapcase,swapcase!,concat,casecmp,start_with,end_with
Merge pull request #1189 from h2so5/mruby-string-ext-addition

Add mruby-string-ext methods
use ISXDIGIT instead isxdigit on parser.y:3485
Refactor kernel.c and class.c
Merge pull request #1190 from lateau/master

tiny fix on parser.y: change isxdigit to ISXDIGIT
Merge pull request #1191 from h2so5/refactor-kernel-and-class

Refactor kernel.c and class.c