adds a .gitkeep file to the controllers, models and services folders

empty folders controllers and models won't be commited on an initial git commit

the missing folders can result in a error when another user clones the repo or after a ```git reset --hard```

```
sails generate api user


/opt/local/lib/node_modules/sails/node_modules/reportback/node_modules/node-switchback/lib/index.js:56
			else throw new Error(err);
			           ^
Error: 'Sorry, this command can only be used in the root directory of a Sails project.'        (`invalid` case triggered, but no handler was implemented.)
    at Function.unknownCase (/opt/local/lib/node_modules/sails/node_modules/reportback/node_modules/node-switchback/lib/index.js:56:15)
    at Function.invalid (/opt/local/lib/node_modules/sails/node_modules/reportback/node_modules/node-switchback/lib/index.js:95:20)
    at Object.module.exports [as before] (/opt/local/lib/node_modules/sails/node_modules/sails-generate/node_modules/sails-generate-controller/lib/before.js:55:13)
    at generate (/opt/local/lib/node_modules/sails/node_modules/sails-generate/lib/generate.js:75:12)
    at module.exports (/opt/local/lib/node_modules/sails/node_modules/sails-generate/lib/index.js:74:2)
    at /opt/local/lib/node_modules/sails/bin/_generate-api.js:34:4
    at /opt/local/lib/node_modules/sails/node_modules/async/lib/async.js:511:21
    at /opt/local/lib/node_modules/sails/node_modules/async/lib/async.js:227:13
    at /opt/local/lib/node_modules/sails/node_modules/async/lib/async.js:111:13
    at Array.forEach (native)
```