Implemented full-text search.

You can now use the search bar on the conversation list to find
conversations, messages, and contacts.

res/drawable/header_search_background.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item>
+        <shape android:shape="rectangle" >
+            <solid android:color="@color/grey_200"/>
+        </shape>
+    </item>
+    <item android:top="1dp">
+        <shape android:shape="rectangle" >
+            <solid android:color="@color/white" />
+        </shape>
+    </item>
+</layer-list>