Members export gives 404 error #51

Hi,

When you try to export a large number of members with many columns, it's possible that the HTTP GET exceeds the maximum length limit. This causes the Umbraco manager to report a 404 error.

I think it's easy to fix by turning the function GetExportedMembers in /Controllers/ExtendedMemberController.cs into a HTTP POST. And probably making a small change to the angular code.

Thanks for looking into it.
Kind regards
Pieter