Fix bug with uploading files on KitKat

This commit fixes #81. The bug was caused by Proguard messing with the
code. This commit adds a simple Proguard rule that prevents it from
doing so.

proguard-project.txt

@@ -35,6 +35,13 @@
 -keep public class * extends android.preference.Preference
 -keep public class com.android.vending.licensing.ILicensingService
 
+# Without this rule, openFileChooser does not get called on KitKat
+-keep class acr.browser.lightning.LightningView$LightningChromeClient {
+	void openFileChooser(android.webkit.ValueCallback);
+	void openFileChooser(android.webkit.ValueCallback,java.lang.String);
+	void openFileChooser(android.webkit.ValueCallback,java.lang.String,java.lang.String);
+}
+
 -keepclasseswithmembernames class * {
     native <methods>;
 }