Make sound cloud embed responsive

When you use a fixed width for the soundcloud embed, it's either going to be too big on the iPhone or too skinny on a large screen. So I changed the width to 100%, which means the full width of its containing element, then put it inside a responsive div whose width (number of columns) changes depending on device width. 

Bootstrap lets you specify multiple column widths for an element, i.e. you can put both .col-xs-6 and .col-md-4 on the element and only one will be used, depending on what the width of the viewport is. In this example, on an extra small device ( < 768px), the element would be 6 elements wide and on a medium device (between 992 and 1200px), it would be 4 columns. If there is not a specification for the particular size, it will use the specification for the next smaller device, and if there isn't any such, the default will be 12 columns (full width). In this example then, a small device ( between 768 and 992px) will also use the xs spec (6 columns) and a large device ( > 1200) will use the md spec (4 columns).

There's also an offset spec, so on extra small devices I use a (left-hand) offset of 1 (column) and width of 10 columns. This centers the embed in the 12 column space (it is not necessary to explicitly specify the right-hand offset, which would be 1 in this case). On small devices I use an offset of 3 columns and a width of 6 columns, and for medium and large devices, offset 4 and width 4. 

You can see the effect of this at http://chetstone.github.io by dragging the corner of your browser  to change the size of the viewport. If you use Chrome it will tell you how many pixels wide the viewport is as you drag it. If you start at the smallest and enlarge the window, you will see the soundcloud embed get wider until one of the breakpoints is reached (768 or 992px), then it will jump back to a narrower size and start getting wider again. 

In order to use columns like this, I had to split the row to put the embed in its own row. I removed some explicit breaks and I think the vertical spacing still looks good. 

I also rounded the corners of the email input form. It looked a little weird with square corners on the left and round corners on the button on the right.