Add withXxxxx() methods to AbstractUser

[mdeknowis]

As discussed in #272 adding the missing with* methods to AbstractUsers

[gmessner]

@mdeknowis
Reviewing this pull request made me realize 2 things:

1. AbstractUser is not marked abstract, and it should be.
2. the withXxxx() methods do not work on a base class. The withXxxx() methods are meant to be chained together, which will not work on the sub-classes of AbstractUser. They can be chained but cannot be assigned to a sub-class without casting. This is why the User class has implemented the withXxxx() methods.

An example trying to use with a sub-class:

Assignee assignee = new Assignee()
    .withAvatarUrl("https://asdad.com/qqwwe")
    .withBio("https://wqeqwqew.com/qweqwe");

Will not compile unless you add a type cast as follows:

Assignee assignee = (Assignee) new Assignee()
    .withAvatarUrl("https://asdad.com/qqwwe")
    .withBio("https://wqeqwqew.com/qweqwe");

What sub-class were you trying to use when you needed the withXxxx() methods?

[mdeknowis]

@gmessner I adjusted the pull request according to your proposal

[gmessner]

@mdeknowis
Sorry about the delay getting to this. Been quite busy the last couple weeks. It's too bad that the Java compiler requires the @SuppressWarnings, but it is worth having this feature so I don't mind them.

Just a couple questions that need to be answered and I'll approve and merge. Thanks again for your contribution.

[gmessner]

@mdeknowis
BTW, As soon as we get this merged, I will also merge the changes that add toString() implementations as per Issue #271

[gmessner]

@mdeknowis
Thanks again for the contribution. I'll let you know when it is released.