Interface and overloaded ComposePrivateMessage to handle captcha.

As suggested by @efskap and @SirCmpwn on SirCmpwn/RedditSharp#49
Forgot to save project file...
Redesign captcha handling.

Abstract class CaptchaSolver handles solving captchas.
CaptchaSolver.HandleCaptcha called with a Captcha object returns a
CaptchaResponse object with necessary data.
Might as well make it an interface again.
Merge pull request #50 from Sharparam/patch-3

Interface and overloaded ComposePrivateMessage to handle captcha.