Removing sniffing in Drag, more info in the PR

Removed:
```javascript
this.selection = (Browser.ie) ? 'selectstart' : 'mousedown';
```
Replaced with:
```javascript
this.selection = 'selectstart' in document ? 'selectstart' : 'mousedown';
```
if the document supports selectstart we will default on that (I think only IE supports that anyway).

Removed:
```javascript
if (Browser.ie && !Drag.ondragstartFixed)
```
Replaced with:
```javascript
if ('ondragstart' in document && !('FileReader' in window) && !Drag.ondragstartFixed)
```
`ondragstart` is now a standard, so the check for the event `ondragstart` is not enough, we have to be a bit more careful here and look if `FileReader` is also present.

Why `FileReader`? Because we should test for `dataTransfer`, but apparently is [_undetectable_](https://github.com/Modernizr/Modernizr/wiki/Undetectables) so we need to check for something else associated with it, and here they suggest to rely on FileReader instead. https://github.com/Modernizr/Modernizr/issues/57#issuecomment-4187079