fix coloWithHexString method

hi, your color library is awesome but contains some bugs.

AVHexColor - colorWithHexString: method
doesn't return valid color object when hex-string that starting with '0' was sent, which is "00ff00" or "000000".

convert char string into numerical hexadecimal value is potentially danger.
so, please consider that merge my-fixed method implementation or refactor it by yourself.

thanks, :)