Project keys update

Allows a specific locale key to be updated

test:
`PUT` to http://localhost:3000/api/projects/53c765424ac9460000aaf55f/keys
```
{
  '_id': '53c765554ac9460000aaf560',
  'key': 'links.home',
  'value': 'titeeeee'
}
```
Didn't we remove Gulp Watch? Or is there something we need this for?