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

composer.json

@@ -7,7 +7,8 @@
   },
   "require": {
     "monolog/monolog": "1.16.0",
-    "doctrine/orm": "2.5.*"
+    "doctrine/orm": "2.5.*",
+    "gedmo/doctrine-extensions": "*"
   },
   "require-dev": {
     "phpunit/phpunit": "*",