Remove capitalization of certain terms within the specification

From Steve Huston's review.

Currently the spec (somewhat haphazardly) capitalizes the initial letter of certain terms such as Connection, Session, Link, Node, etc. in an attempt to indicate that they are referring to AMQP entities. The convention is never explained nor is it applied consistently.

While it may be advantageous to use some typographical convention to identify such terms, it is better to remove the inconsistent application of initial caps that currently exists.