Add compilation levels : simple & whitespace

You can probably remplace the dist/ngeo.js, dist/ngeo-simple.js and dist/ngeo-whitespace.js rules by a single one:

```
dist/ngeo%.js: buildtools/ngeo%.json .build/externs/angular-1.3.js $(SRC_JS_FILES) .build/node_modules.timestamp
        mkdir $(dir $@)
        node buildtools/build.js $< $@
```

I haven't tested it, but I'm confident it will work.

`make` rocks! Curious to know how the grunt hipsters do that :-)