Implement background task for fetching RSS feeds

* Implemented in briar-core as a `ScheduledExecutorService`
  that gets started when the app starts
* The briar-api has a `FeedManager` interface
  that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor

Closes #484

briar-android/proguard-rules.txt

@@ -59,4 +59,14 @@
 -dontnote android.support.**
 -dontnote dagger.**
 -dontwarn dagger.**
--dontwarn com.google.common.**
+-dontwarn com.google.common.**
+
+
+# RSS libraries
+-keep class com.rometools.rome.feed.synd.impl.** { *;}
+-keep class com.rometools.rome.io.impl.** { *;}
+-dontwarn javax.xml.stream.**
+-dontwarn org.jaxen.**
+-dontwarn java.nio.**
+-dontwarn org.codehaus.mojo.animal_sniffer.**
+-dontwarn org.slf4j.impl.**