Experimentations with an elvis operator

This could allow null-safe method invocations, like:

    person: address()?: city()

Calls with ?: instead of : return null if ever done on a null reference:

    null?: plop()

returns null, while:

    null: plop()

throws a NullPointerException.
Elvis tests refactorings
Documentation for null-safe method invocations
An example of dealing with null-safety
Forgot the license header
Merge pull request #9 from jponge/wip-elvis

null-safe method invocations