Retain annotations when using proguard + gson. From patch on issue 358.

Fixes issue 358.

examples/android-proguard-example/proguard.cfg

@@ -5,10 +5,10 @@
 -dontskipnonpubliclibraryclassmembers
 -dontpreverify
 -verbose
--dump class_files.txt 
--printseeds seeds.txt 
--printusage unused.txt 
--printmapping mapping.txt 
+-dump class_files.txt
+-printseeds seeds.txt
+-printusage unused.txt
+-printmapping mapping.txt
 -optimizations !code/simplification/arithmetic,!field/*,!class/merging/*
 
 -allowaccessmodification
@@ -77,6 +77,9 @@
 # removes such information by default, so configure it to keep all of it.
 -keepattributes Signature
 
+# For using GSON @Expose annotation
+-keepattributes *Annotation*
+
 # Gson specific classes
 -keep class sun.misc.Unsafe { *; }
 #-keep class com.google.gson.stream.** { *; }