Fix Checkstyle violations

- Mostly missing Javadoc on large methods
- CommandInitializer: split command building method into multiple methods

src/main/java/fr/xephi/authme/data/limbo/persistence/DistributedFilesPersistenceHandler.java

@@ -114,7 +114,7 @@ class DistributedFilesPersistenceHandler implements LimboPersistenceHandler {
         }
 
         try {
-            return gson.fromJson(Files.toString(file, StandardCharsets.UTF_8), LIMBO_MAP_TYPE);
+            return gson.fromJson(Files.asCharSource(file, StandardCharsets.UTF_8).read(), LIMBO_MAP_TYPE);
         } catch (Exception e) {
             ConsoleLogger.logException("Failed reading '" + file + "':", e);
         }

src/main/java/fr/xephi/authme/data/limbo/persistence/IndividualFilesPersistenceHandler.java

@@ -46,7 +46,7 @@ class IndividualFilesPersistenceHandler implements LimboPersistenceHandler {
         }
 
         try {
-            String str = Files.toString(file, StandardCharsets.UTF_8);
+            String str = Files.asCharSource(file, StandardCharsets.UTF_8).read();
             return gson.fromJson(str, LimboPlayer.class);
         } catch (IOException e) {
             ConsoleLogger.logException("Could not read player data on disk for '" + player.getName() + "'", e);