add "strip" family to String.
add mrbgems/ext/mruby-numeric, and method: Integer#chr
add Integer#chr test case: multibyte (raise RangeError)
fix comment
Fixed #969
Merge pull request #967 from iij/pr-string-strip

add "strip" family methods of String to mruby-string-ext
Merge pull request #968 from kouki-o-iij/pr-mrbgem-numeric-ext

add mrbgems/ext/mruby-numeric, and method: Integer#chr
Merge pull request #970 from masuidrive/fix_building_script

Fixed #969
OP_ARRAY may overflow ARG_C
Merge branch 'master' of github.com:mruby/mruby