resolve error while exporting special characters (e.g. æøå)

When trying to export to excel and csv the log contained errors due to the study description containing special characters (norwegian) that were not correctly converted into the ascii character set.

This patch fixed that by encoding all unicode strings as utf8 before writing.

Still couldn't export to excel due to multiple xrayfilters: (Exception: get() returned more than one XrayFilters -- it returned 2!). But this might be fixed in development, server is running 0.7.4)

CSV now outputs utf8 encoded files. Although ÆØÅ is included in standard 8-bit ascii, using utf-8 should work with more exotic characters as well.