Improve appearance of vector markers.

None of the current vector marker styles resembles a standard vector marker
as found in other scientific plotting applications. In addition, due to the
small size of the vector markers, they tend to clutter the image too much
and the direction might not be easy to grasp.

The `ImageProducer` class already has an option to control the size of the
vectors, whose value is 1 by default, and which scales the marker by that
factor in both directions. The spacing between vectors is adjusted so that
origins of vectors are not overlapped.

So this request proposes a new style parameter `vectorScaleFactor`,
directly mapped to that factor, and a new a vector style `prettyvec`
with a standard arrow shape:

  * Add `prettyvec` vector style to display pretty vector markers,

  * Expose the vector scale factor option as a style parameter
     to control the density of vector markers in the rendered image.

  * Change vector marker for style `vector` (vectors above pseudocolor-map)
    from `stumpvec` to new `prettyvec`.

  * Fill vectors in `fancyvec` style, as done in all other vector styles.

  * Fix listing of vector styles in jsp files.

The results can be tested at SOCIB's web map viewer
(layer parameters are on the Style Selection panel on the top right corner):
  http://thredds.socib.es/lw4nc2/?m=satwind&i18n=en_EN

![screenshot](https://cloud.githubusercontent.com/assets/3691987/4186327/d75629fe-375f-11e4-8456-edd344ee2fd6.png)