added variable references to assets directory

Adds the ability to specify the location of the assets directory when generating a new sails project.  For example, the command:

```bash
mkdir myproject && cd myproject
sails new server --assets-location '../client'
```
Generates a folder structure like this:
```
myproject
  + client
  | - images
  | - js
  | - styles
  |   favicon.ico
  |   robots.txt
  + server
  | - api
  | - config
  | - node_modules
  | - views
  |		.gitignore
  |   .Gruntfile.js
  |   .README.md
  |   app.js
  |   package.json
```

I've closed this [issue](https://github.com/balderdashy/sails/issues/1168) I opened since this pull request resolves it.