Add fluent API to PulsarTemplate

[onobc]

See #68

@sobychacko @alpreu
I am still narrowing down the choice of fluent method styling.

More naturally readable ('with' on suffix - current choice)

pulsarTemplate
    .newMessage("foo")
    .toTopic("topic-1")
    .routedWith(fooRouter)
    .customizedWith(fooKeyCustomizer)
    .sendAsync();

More typical builder style ('with' on prefix)

pulsarTemplate
    .newMessage("foo")
    .withDestinationTopic("topic-1")
    .withCustomRouter(fooRouter)
    .withMessageCustomizer(fooKeyCustomizer)
    .sendAsync();

More brief/terse

pulsarTemplate
    .newMessage("foo")
    .to("topic-1")
    .router(fooRouter)
    .customizer(fooKeyCustomizer)
    .sendAsync();			

Wdyt?

[alpreu]

I like the 'more typical builder style' as well as 'more brief/terse' but this is just personal preference :)