Port SimpleImmutableEntry for Android2.2 or below

msgpack-java was crashed on Android 2.2 or below when my application called SequentialMapValueImpl$EntrySetIterator.next(). It is because the method calls java.util.AbstractMap$SimpleImmutableEntry that doesn't exist in Android 2.2 or below.

Then I ported java.util.AbstractMap$SimpleImmutableEntry as org.msgpack.util.PortedImmutableEntry. I saw this commit worked on both Android 2.2 and 4.2.