FLUME-2511. Allow configuration of enabled protocols in Avro source and RpcClient.

(Tom White via Hari)
Preparing for Flume-1.5.1 Release
FLUME-2441. HTTP Source Unit tests fail on IBM JDK 7

(Li Xiang via Hari)
FLUME-2533: HTTPS tests fail on Java 6

(Hari Shreedharan via Jarek Jarcec Cecho)
Updating CHANGELOG for 1.5.1 release