Add cache for Posts that is Shell-aware and throws Shells out if possible.

diff --git a/src/main/java/net/pterodactylus/sone/core/PostCache.java b/src/main/java/net/pterodactylus/sone/core/PostCache.java
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+++ b/src/main/java/net/pterodactylus/sone/core/PostCache.java
@@ -0,0 +1,66 @@
+/*
+ * Sone - PostCache.java - Copyright © 2010 David Roden
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package net.pterodactylus.sone.core;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import net.pterodactylus.sone.data.Post;
+import net.pterodactylus.sone.data.PostShell;
+import net.pterodactylus.sone.data.Shell;
+
+/**
+ * {@link Shell}-aware {@link Map} from post IDs to {@link Post}s that exchanges
+ * an existing {@link Shell} against the real object once it’s available.
+ *
+ * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+ */
+public class PostCache {
+
+       /** The posts. */
+       private final Map<String, Post> posts = new HashMap<String, Post>();
+
+       /**
+        * Stores the given post in this cache. If the given post is not a
+        * {@link Shell}, it is stored. If it is a {@link Shell} it is only stored
+        * if there is no post stored for the ID of the given post. If the given
+        * {@code post} is a {@link Shell}, it needs to have its
+        * {@link PostShell#setId(java.util.UUID) ID} set!
+        *
+        * @param post
+        *            The post to store
+        */
+       public void store(Post post) {
+               if (!(post instanceof Shell<?>) || !posts.containsKey(post.getId())) {
+                       posts.put(post.getId(), post);
+               }
+       }
+
+       /**
+        * Returns the post with the given ID.
+        *
+        * @param id
+        *            The ID of the post
+        * @return The post with the given ID, or {@code null} if there is no post
+        *         with the given ID
+        */
+       public Post get(String id) {
+               return posts.get(id);
+       }
+
+}