Create tests for line-symbolizer, using expressions for parameters

- there are two versions:
  - line-symbolizer-expressions: stroke, stroke-width, stroke-opacity, offset
  - line-symbolizer-expressions-all: additionally test
    stroke-dasharray, stroke-linecap, stroke-linejoin, comp-op
    which are not supported right now

Attached are the images as they get rendered on my installation. The line-symbolizer-expressions-all images don't get rendered.

![line-symbolizer-expressions-256-256-1 0-agg](https://f.cloud.github.com/assets/249012/2129434/55b8d838-9287-11e3-9d45-d325aa5fbc35.png)
![line-symbolizer-expressions-256-256-2 0-agg](https://f.cloud.github.com/assets/249012/2129435/55ba9038-9287-11e3-98fd-f9a093e1116b.png)
![line-symbolizer-expressions-256-256-1 0-cairo](https://f.cloud.github.com/assets/249012/2129436/55bb464a-9287-11e3-922f-0ea2a61b661f.png)
![line-symbolizer-expressions-256-256-2 0-cairo](https://f.cloud.github.com/assets/249012/2129437/55be14a6-9287-11e3-85f0-673e6552280c.png)

Interestingly stroke-width is missing in the Cairo version. Also I noticed the difference in the reference images for line-symbolizer-expressions-all for comp-op - is this expected?