Main page: detect platform and offer the right setup file

Works but needs some cleanup.

If anyone has any thoughts let me know. You can see the live example on mpc-hc.org.
This is kinda duplicated, why not do;
```
if (arch === "x64") {
  folder = "MPC%20HomeCinema%20-%20x64";
...
element.attr("href", url + folder + "/MPC-HC_v" + version + "_" + arch + "/...and so on)
```