Replace plusp

Because plusp is cl function, it requires loading 'cl.el' or 'cl-lib.el.
Merge pull request #1 from syohex/remove-cl-function

Replace plusp
Update readme
Merge remote-tracking branch 'remotes/origin/master'
Use string-to-number instead of string-to-int

string-to-int is deprecated and it causes the byte compile warning.
Merge pull request #2 from syohex/replace-deprecated-function

Use string-to-number instead of string-to-int