Adds MotD and Message model as well as tests

This commit adds the MotD model for storing and retrieving
messages-of-the-day.

It also adds a model for messages. Messages have an author as well as a
thread they belong to, the thread can be read by a specific number of
authors. FUrthermore, the message model supports system messages.

This commit introduces a number of needed changes:
- Character is now implementing the CharacterInterface
- MissingCharacter and SystemCharacter are supporting "pseudo-characters"
- trait MockCharacter implements non-implemented methods for
  MissingCharacter and SytemCharacter
- Characters are now soft-deletable. Models wanting to load soft-deleted
  characters need to fetch them eagerly.

Closes #17

tests/datasets/motd.yml

@@ -0,0 +1,43 @@
+characters:
+    -
+        id: 1
+        name: "Testcharacter 1"
+        displayName: "Testcharacter 1"
+        health: 0
+        maxhealth: 100
+    -
+        id: 2
+        name: "Testcharacter 2"
+        displayName: "Testcharacter 2"
+        health: 90
+        maxhealth: 90
+    -
+        id: 3
+        name: "Testcharacter 1"
+        displayName: "Deleted Testcharacter"
+        health: 200
+        maxhealth: 200
+        deletedAt: "2011-11-11 11:11:11"
+motd:
+    -
+        id: 1
+        author_id: 1
+        title: "This is the title"
+        body: "This is the body of the message"
+        creationTime: "2016-05-03 14:19:12"
+        systemMessage: false
+    -
+        id: 2
+        author_id: 1
+        title: "This is a system message"
+        body: "This is the body of the system message"
+        creationTime: "2016-05-04 14:19:12"
+        systemMessage: true
+    -
+        id: 3
+        author_id: 3
+        title: "This is an old message."
+        body: "This is an old message."
+        creationTime: "2002-12-09 15:13:59"
+        systemMessage: false
+