Merge pull request #84 from guemon/master

Fix typo
Add description of ISDIGIT
Fix optparse/time example
Merge pull request #87 from mneilsen/master

Fix optparse/time example
Merge pull request #86 from utenmiki/update-ISDIGIT

Add description of ISDIGIT
add Module#singleton_class? since 2.1.0
add {exception: false} optional argument to IO#read_nonblock and #write_nonblock since 2.1.0
fix typo