Fix: Word export encoding was said to be latin, but is in fact UTF-8

Dev: by default Word uses local 8-bit encodings (CP-1252 for Europe etc.) but LimeSurvey exports UTF-8 without specifying the encoding in the resulting document. This patch adds a tag setting charset to UTF-8 in the resulting .doc. It also corrects the label of the export option from latin to UTF-8.

I tested this fix with Word 2002, Word 2010, Libre Office 3 and Libre Office 4 : all seem to take into account the charset tag I added and display the document correctly.

Before :
![before](https://f.cloud.github.com/assets/3476089/1391246/fc64d4e6-3bf9-11e3-9734-e80c8c128fc7.png)
After :
![after](https://f.cloud.github.com/assets/3476089/1391251/0558dad4-3bfa-11e3-92ef-74d86cb7fd40.png)