Partial support for ODT export

Implemented partial support for ODT export:

+ Container:
  + Supported:
    + Colors match CSS
    + Resize in height if content gets to big
    + Picture is present in ODT also
    - Picture is not vertically centered
    - Picture is re-sized if the box is re-sized e.g. in OpenOffice Writer
+ Spans:
  + Supported:
    + Colors match CSS
    + Styles match CSS (e.g. subscript, emphasis)
    - Small picture is missing
    - Alignment and width are ignored
- The rest is not supported right now

Columns are currently not supported. The reason is the missing start and end "markers" of the first and last column. I think it could best be implemented using a table but a table has an opening and closing tag in ODT format. But the syntax doesn't tell me what the last "<WRAP column>...</WRAP column>" pair is.