Added support for discovering injectable ("Burstable") fields.

Implements square/burst#36.
Maybe combine the first and third branches? something like

    if (constructor.getParameterTypes().length > 0 && fields.length > 0) {
        throw IllegalStateException(
            "Class "
                + cls.getName()
                + " has a non-default constructor, so cannot also be parameterized on fields");
    }
    filteredConstructors.add(new TestConstructor(constructor, fields));