Changing particle parameters to kivy properties to enable real-time editing

Hi skitoo,

I've been working a bit more on our editor for your particle system, and as Jacob mentioned we needed to make a few changes to engine.py in order to make all the parameters change in real time. Basically, we made all the relevant parameters into Kivy properties, which was enough to make the particles responsive to changes in almost all situations. Two of the parameters, max_num_particles and texture were a bit more complicated, and we had to add short listeners to these parameters to make the changes happen in real time. Let me know if you're interested in merging these changes with your particle system; otherwise we will have to subclass ParticleSystem in our editor.

Thanks for all the awesome work; the system is super fast and pretty!

Sam