Add toast-top-center / issue#75

Hello! 
I made a "toast-top-center", but I'm not sure it is "toast-center" you said at #75 
I just added `toastr.css` and `demo.html` to confirm this change.
Also, I don't know whether the width 300px is the best for toast-top-center, please optimise it.

Thanks, Shogo.

#This is my first PR, so might have some clumsiness.
#And, I'm sorry for my poor English.