Fix  #656 add font_size in create_captcha()

Fixed 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.
    ($font_size >= 5) && $font_size = 5;