FR: $mdToast 'position' should suport rtl

[amitport]

Currently, we can specify left or right for the $mdToast position attribute.

In web pages that has both rtl and ltr versions it is sometimes necessary to specify start/end position instead of left/right. Those should specify the position in relation to the the container direction. That is, when direction=ltr then start=left,end=right, and when direction=rtl, it's start=right,end=left.