Add a strokeLinejoin for vector styling

Some vector layers with sharp edges like GPS track look very ugly with the library default `PdfContentByte.LINE_JOIN_MITER` 
![print-kml-linejoinmiter](https://f.cloud.github.com/assets/1236739/1899077/3d05bdb6-7c33-11e3-847a-7a6f9a276311.png)

This PR propose to add a `strokeLinejoin` vector style property, similar to the existing `strokeLinecap` one.
So defining the above example with `"strokeLinejoin": "round"`, we will improve the rendering.

![print-kml-linejoinround](https://f.cloud.github.com/assets/1236739/1899105/c71aa30e-7c33-11e3-8cf9-05f0bee3305e.png)

Possible values for `strokeLinejoin` are `miter` (default), `round` and `bevel`.