v2.8.0-alpha.1

Release of 2.8.0-alpha.1

• Add Android 5.1 support.
• Enable auto delete of pending of message after a configured number of retry is exceed.
• Brought back HTTP client support for devices running Android v2.2.
• Fix inconsistency with message result API codes.
• Fix issue with sync to only one Sync URL even though multiple have been enabled.