Lots of updates: class interface, plotting tools, documentation, iPython notebook

Hi Christophe, 

I found some time on Friday and today to work on the python MCFOST tools. Here's a bunch of updates. In particular I've implemented an object class for entire model results directories (parameter file + SED + images for one model) with SED plotting and image display, and started a similar interface for observational data.  I've also expanded the documentation a bit and added an iPython notebook demonstrating some of the new code. 

Let me know when/if you get a chance to test any of this.