add support for sending/receiving message of amqp sequence type

[yunhaoling]

• Based on the option1 design proposal: https://gist.github.com/yunhaoling/b7c30894f59542c70de589940a319460
• Added support for AMQP Sequence as the body type of the message.
• Added new class uamqp.MessageBodyType to denote the body type of an amqp message, including:
  • DataType: The body consists of one or more data sections and each section contains opaque binary data.
  • SequenceType: The body consists of one or more sequence sections and each section contains an arbitrary number of structured data elements.
  • ValueType: The body consists of one amqp-value section and the section contains a single AMQP value.
• Added new parameter body_type to the constructor of uamqp.Message which takes uamqp.MessageBodyType to denote the body type of an amqp message.
• Added support for sending/receiving message whose body consists of sequence into the c uamqp library