Wrongly oriented image files generated from landscape PDFs

I have a PDF file exported from Powerpoint that c.documentviewer wrongly converts to images in portrait mode. The pages of the source PDF are all in landscape.

My fix for this issue was to change the order of the GM options in converter.py:

from:
```
cmd = [
self.binary, "convert",
'-resize', str(size[1]) + 'x',
'-density', '150',
'-format', format,
filename, output_file]
```
to:
```
cmd = [
self.binary, "convert",
filename,
'-resize', str(size[1]) + 'x',
'-density', '150',
'-format', format,
output_file]
```

The gm [documentation](http://www.graphicsmagick.org/convert.html) states:

`gm convert [ options ... ] input_file [ options ... ] output_file
`

so encoder options are better added after the input_file