Wpfcst

Hi Alex - I hope I get this pull request thing right... wpfcst branch contains the route forecasting logic we had discussed via email.  It takes a Dataset object with forecast data, combines it with route data read from a file (currently simple csv) and produces a gpx output that can be loaded into OpenCPN as a temporary layer.  I have broken up the additions/changes  into four commits:

1. **497ef83** - rtefcst.py under sl/lib/ and associated test module under test/.  Module doc string should get you started.

2. **24fa7a0** - Some smaller changes and additions to sl/lib/objects.py.  See "Notes to Alex" in comments for details/rationale.

3. **4d4eb01** - Additions/changes to slocum.py.  Again, "Notes to Alex" in comments will have details.

4. **d46d927** - I added a 'files' directory with OpenCPN waypoint icons for wind arrows.  See README in that directory for additional info.

The task handler for rtefcst in slocum.py currently creates the forecast Dataset from a windbreaker packed forecast.  Ultimately, this could also be a normal GRIB or be called directly from some module that can pass in a Dataset with forecast (if run on server).

    python slocum.py rtefcst --help 

Should provide enough detail in case you want to try it.  

I also put a [post on the NX discussion board](http://navigatrix.net/viewtopic.php?f=16&t=601) to get some feedback regarding possible request formats.

Cheers,
Markus