Create and use EiMessageHeaderAccessor

In the move to being part of core Spring, a number of constants and
header accessor methods have moved from MessageHeaders to
MessageHeaderAccessor. This commit continues this pattern for
the enterprise integration headers that are specific to Spring
Integration. A new class, EiMessageHeaderAccessor, has been created.
This class provides constants and methods for working with SI-specific
headers. The main code and tests have been updated to use this new
class.
Prevent Rome from pulling in old version of Xerces

In a recent update (60532cbd), Spring's started calling 
DocumentBuilderFactory.setFeature(). This method isn't implemented by
the version of Xerces that Rome depends upon, resulting in
AbstractMethodErrors being thrown. To overcome this, the Xerces
dependency has been excluded when declaring the dependencies on Rome.