ADD: Support IE8,9 for CORS

This commit should work for IE(8 -11)

Example
```
var sagent = require('superagent')
sagent.post('/example')
  .crossDomain(true)        // Add this line to support cors in IE8 and 9
  .end(function(err, res){
    
  })
```
The following is done to fix the issue
- IE 8-9 support XDomainRequest instead of XMLHTTPRequest
- IE 8-9 has an issue of making too many request to same domain would cancel out each other. Therefore, a mechanism is introduce to prevent firing too many request at the same time
By default, any header is strip off when the request is fired in IE8 and 9. So no authentication and other thing related to header is supported