change Submit / add Captcha functions

changed parameters for Submit as subreddit no longer uses the submitter
interface, added captcha support
add tests for captcha
add CaptchaImage

gets an image that corresponds to the captcha iden returned by
NewCaptchaIden
remove function from the submitter interface

submitter interface no longer used
add captcha support / remove submitter interface

submissions can now be made that require captcha