Captcha should be configurable

Currently captcha is 'a'-'z' and non-configurable
This should be improved:

following attributes should be added to language.xml:

1. 'tip' - short text describing this captcha for ex. "Enter uppercase English letters"
2. 'rangeStart' - initial letter/code of possible captcha characters
3. 'rangeEnd' - final letter/code of possible captcha characters

'tip' above should be displayed as placeholder