Add a Thor command to install a widget by its Gist Id.

Hi everyone. 
This is a commit to let a Dashing user install a widget by its Gist Id (i.e ```4990174``` for the [weather](https://gist.github.com/davefp/4990174) widget).
It's pretty basic so don't hesitate to ask for improvement if you have something precise in your mind.

### Usage

```dashing install 4990174```

### How it's working 

It gets all the files from the Gist repo and for each of them except the ```*.md```
- if it's ```*.rb``` copy it in the ```jobs``` folder
- if it's ```*.html | *.scss | *.coffee``` create a directory under ```widgets``` and copy those files inside.