adding .gitignore

.gitignore

+# built application files
+*.apk
+*.ap_
+
+# files for the dex VM
+*.dex
+
+# Java class files
+*.class
+
+# Mac files
+.DS_Store
+
+# generated files
+bin/
+gen/
+
+# Ignore gradle files
+.gradle/
+build/
+
+# Local configuration file (sdk path, etc)
+local.properties
+
+# Proguard folder generated by Eclipse
+proguard/
+
+# Eclipse prefs files
+.prefs
+
+# Android Studio
+*.iml
+.idea
+.gradle
+build/
+
+#NDK
+obj/