#930 Create registration captcha manager

- Introduce registration captcha manager, rename login captcha manager accordingly
- Integrate reg. captcha manager into /register command

Open points:
- Refactor common captcha functionality into abstract superclass
- If captcha before /register necessary, show appropriate message to player immediately
- Unit tests

src/main/java/fr/xephi/authme/message/MessageKey.java

@@ -134,6 +134,9 @@ public enum MessageKey {
     /** Captcha code solved correctly! */
     CAPTCHA_SUCCESS("valid_captcha"),
 
+    /** To register you have to solve a captcha code first, please use the command: /captcha <theCaptcha> */
+    CAPTCHA_FOR_REGISTRATION_REQUIRED("captcha_for_registration", "<theCaptcha>"),
+
     /** A VIP player has joined the server when it was full! */
     KICK_FOR_VIP("kick_forvip"),