Rename scripts -> framework; assets -> interactives

This makes it clearer what is in each folder: framework contains
the populations.js framework, and interactives will contain all
the individual authored files (and assets needed).
Compile .coffee in interactives/ **requires npm install**

This is so that we can author individual models in CS in the interactives folder and compile it to individual JS files in 
public/.

There doesn't seem to be a native brunch way to do this, so 
this uses the (kinda hacky) after-brunch plugin to execute 
arbitrary commands after each compile.

Since the .coffee files were already being brought over from
interactives/ intact (since interactives/ is now the equivalent
of assets/) it would have been nice to simply compile them
in-place. However, the coffee compile doesn't seem to be able
to do this. Further, when I tried to compile and then delete

    coffee --compile public
    find public/ -type f -name "*.coffee" -delete

the delete command would happen faster than the compile command. 
The only solution seems to be to remove the existing .coffee 
files in public/ and compile the ones in app/.
Initial refactoring of species and model scripts

Species can now be shared between different models, and different
html files can take different model scripts.

The model script will need to be refactored further, but this is
the minimal step without changing the existing script too much.