Provide public setMinTime() and setMaxTime() methods

[nickdaugherty]

After instantiation, it's no longer possible to set the min/max times (for example, when using 2 DateSliders as a range picker). The DateSlider class provides a setTime() method, it should also provide setMinTime() and setMaxTime() to allow changing those values on the fly. I've added those methods to my own code and they seem to work fine:

public void setMinTime(Calendar c) {
    minTime = c;

    if(c != null){
        mContainer.setMinTime(c);
    }
}

public void setMaxTime(Calendar c) {
    maxTime = c;

    if(c != null){
       mContainer.setMaxTime(c);
    }
}