Add first basic implementation of Apgservice

Provides an AIDL-API for other apps to encrypt and decrypt a string
symmetrically with a passphrase. Function names and API is by no way
finalized and will change!

Support for asymetric encription will follow.

For reference and discussion see issue #71,
https://code.google.com/p/android-privacy-guard/issues/detail?id=71

src/org/thialfihar/android/apg/IApgService.aidl

@@ -0,0 +1,6 @@
+package org.thialfihar.android.apg;
+
+interface IApgService {
+    String encrypt_with_passphrase(String msg, String passphrase);
+    String decrypt_with_passphrase(String encrypted_msg, String passphrase);
+}