Issue #330 Implementing several minimal features in PactDsl

[lordofthejars]

Implementing several minimal features in PactDsl.

• implements a .headers(String... headers) so instead of having to create a separate Map, you can do something like .headers("content-type", "application/json")
• implements a .body(java.util.Consumer<>) method. Now users can only use JSONObject, Document or String, but if we create a functional method then users wil be able to use lambda to for example transform Jackson POJO to String or JSR 364 JsonObject.
• implements a toJson method that allows users to set Json/xml as string using single quotes, so avoiding having to skip over and over again the (") char. https://github.com/arquillian-testing-microservices/game-service/blob/contract-tests/src/test/java/org/arquillian/microservices/gameservice/AgeCheckerConsumer.java#L82

[lordofthejars]

One question does this PR also rebased to 3.5 beta too?

[uglyog]

I'll merge master into v3-spec branch, which 3.5.x is built off.