Handle viewing of attachments that are, themselves, emails

* Add intent filter for application/eml and message/rfc822 mime types,
  launching MessageView with a Uri
* Modify loadMessageTask to handle the Uri by parsing the attachment's
  input stream with Pop3Message.parse(), and then creating an
  EmailProvider message in a special Mailbox created to hold
  "attachment" messages
* Delete all "attachment" messages after the parent message is closed
* Add unit tests

Change-Id: I20276ee006b9f05b889f3c808d3dc407cde26d49
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index e982ec6..c19863d 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -191,6 +191,12 @@
         <activity
             android:name=".activity.MessageView"
             android:theme="@android:style/Theme.NoTitleBar" >
+            <intent-filter android:label="@string/app_name">
+                <action android:name="android.intent.action.VIEW" />
+                <data android:mimeType="application/eml" />
+                <data android:mimeType="message/rfc822" />
+                <category android:name="android.intent.category.DEFAULT" />
+            </intent-filter>
         </activity>
         <activity
             android:name=".activity.MessageCompose"