closing  #656 add font_size in create_captcha()

closing Issue #656 

Without using TTF Font,  the maximum size of font accepted by imagestring is 5 (which is default as well). With TTF font I am setting an upper limit for font size as 30. The default font size is 16. 

This is my first pull request. Please let me know if I am missing anything.
Same as https://github.com/bcit-ci/CodeIgniter/pull/3325/files#r20223592