list export: adding the 'to_xls' gem.

This gem will handle export to native xls files, based on the
‚spreadsheet‘ gem.
adding basic ListExport class.

This will handle CSV and XLS exports and possibly others.

Trello: https://trello.com/c/xYrNDwgv/581-csv-listen-export
list export: Implementing birthday list export.

I’ve used the existing one `app/models/group_mixins/csv` as a template.

The new one is implemented in the ListExport class.

After migrating all presets, the controller has to be adapted to use
the ListExport class and the old specs and csv additions have to be
removed.
list export: migrating the member_names list.
list export: migrating address_list.
list export: migrating the phone_list.
list export: migrating email_list.
list export: migrating member_development list.
list export: some clean-up.
list export: switching groups#show for CSV and XLS to the new ListExport mechanism.
list export: including the xls tables in the download menu of groups#show.
list export: include excel mimetype icon.

this icon has been adapted from this resource, where it was decleared as free.
http://icons.iconarchive.com/icons/carlosjj/microsoft-office-2013/128/Excel-icon.png

http://www.iconarchive.com/show/microsoft-office-2013-icons-by-carlosjj/Excel-icon.html

LICENSE

License: Free for non-commercial use.
Commercial usage: Not allowed
The products or characters depicted in these icons are © by Microsoft
list export: Spalte Bezirksverband in Wingolfsplattform ergänzen für: Address-Listen, Geburtstags-Listen, Namenslisten; allgemein für alle Listen, in denen Benutzer die direkt aufgelisteten Objekte sind.
Merge pull request #54 from fiedl/sf/list_export

New list export mechanism including CSV and native XLS.

Ich habe gleich noch mit die BV-Spalte eingefügt, da diese für einen aktuellen Export im Karlsruher Wingolf benötigt wird.