CORS & Custom header support

Added ability for the following:

- Passing custom headers
- Pass xhrFields

Usage:

    elfinderOptions = {
        customHeaders: {'X-Requested-With': 'XMLHttpRequest'},
        xhrFields: {
                withCredentials: true
        },
        customData: anyCustomData,
        url : theURl,
        ...,
        uiOptions,
        ...
        and so on..
    };
            
    elf = $("#elfinder").elfinder(elfinderOptions);