Add aes128gcm

* Update the HTTP Encrypted Content Encoding implementation to support
aes128gcm, as defined in the 9th version of the draft [1].
* Update the Web Push implementation to use aes128gcm, as defined in the
9th version of the draft [2].

[1] https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-09
[2] https://tools.ietf.org/html/draft-ietf-webpush-encryption-09

Author: martijndwars

.gitignore

@@ -3,3 +3,4 @@ build/
 .gradle/
 *.iml
 target/
+out/

CHANGELOG.md

@@ -0,0 +1,5 @@
+# 4.0.0
+
+* Support [aes128gcm content encoding](https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-09#section-2)
+  * Use `PushService.send(Notification, Encoding)` or the analogous `sendAsync` with `Encoding.AES128GCM`.
+

src/main/java/nl/martijndwars/webpush/Encoding.java

@@ -0,0 +1,5 @@
+package nl.martijndwars.webpush;
+
+public enum Encoding {
+    AESGCM, AES128GCM
+}