Captcha classes. Contains :

- a CaptchaSecurityContext that overrides the default SecurityContext and holds some captcha related informations
- an abstract CaptchaChannelProcessorTemplate and its implementations that test this context according to the configuration
- a CaptchaServiceProxy and a CaptchaValidationProcessingFilter that alows to validate a captcha response and to update the CaptchaSecurity
- a CaptchaEntryPoint that redirects to a captcha page if the CaptchaChannelProcessor implementation decide so