Customizable message CSS classes

Hello, idea of this change is ability to specify what CSS classes I want to use for my messages.
Right now classes are hard-coded to bootstrap.
But it's common to use other frameworks and, since I can't change code of a framework, I want to manually wire my message classes to them.

Also in this commit I provided some tests due to previous lack of such.

No breaking changes, all classes are set bootstrap in default.