New list export mechanism including CSV and native XLS.

In this PR, I'm refactoring the CSV export mechanism. These are the key points:

* The exort methods are now wrapped within a `ListExport` class.

  ```ruby
  # Controller
  send_data ListExport.new(User.all).to_xls, ...
  ```
* Currently, `to_csv` and `to_xls` are supported.
* Since, this also accepts Arrays and ActiveRecord-Relations of Users (not only Groups), this mechanism can be used to export search results later.
* This mechanism can be extended by and `to_html` method for later use with jquery datatables.
* The wrapper class allows local additions, e.g. to add the `bv` column in Wingolfsplattform.
* CSV files now send an utf-8 byte order mark.

The export menu looks like this:

![bildschirmfoto 2014-08-21 um 19 39 03](https://cloud.githubusercontent.com/assets/1679688/4000484/1bcf19c4-295a-11e4-8703-b5abb98a2d2b.png)