Polish spring-ws-support's dependencies

Use separate API and implementation dependencies for JavaMail. This
paves the way for JavaMail 1.5 where javax.mail:mail is no more.

Exclude the mail dependency from mock-javamail to avoid having multiple
copies on the classpath

Exclude the Geronimo JMS spec from ActiveMQ as the official
java.jms:jms-api artifact is already available
Merge pull request #16 from wilkinsona/polish-dependencies

* polish-dependencies:
  Polish spring-ws-support's dependencies