Make reference and userguide directory hierarchies simpler.

Directory for reference used to be doc/doc/freetds-VERSION/reference.
This is however quite long causing problems when generating tar
distribution files.
Renamed reference to doc/reference (similar for userguide one).

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Output svg files for reference documentation.

SVG are much smaller then png. There is no need user have to download
larger file just for development stuff (tarball increase size from 2 to
30 mb)

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>