Start printing out excel sheets in XML rather than html tables.

Consolidated periods#list, groups#list and teams#list into
assignments/list(.html|.xml). This file sets the xml separators and
prints out assignments/_list_data.xml.

NOTE: Still need to add this feature for reports. It is broken
currently.
Close #25. Finish converting xls to xml.

Also sorting reports by date, not reverse date, on assignments#show
Merge branch 'excel'
Bugfixes for groups#list and teams#list
Bugfix in assignments/_list_data.xml

In order for Microsoft Excel to recognize the xml document, the document
must be fully properly formatted. Furthermore, in order for things to
display correctly, you can't leave any space around number in cells.