Add javadoc code examples.

Hi,

I've take times to report some of assertj-examples in the API javadoc.
There is still a lot to do but it can be done in several times.

A question though, What do you expect for the `@throws` documentation in interface implémentation ?
As `{@inheritDoc}` does not inherits `@throws` section automatically, you have to redeclare them in the implementation.
The problem is that you should not duplicate '@throws' when throwing the same exception. Using `@throws AssertionError {@inheritDoc}` will only generate documentation from the first occurence of the implemented interface...
Do you want me to copy/paste them anyway ?

Thanks,