add some gulp tasks: lint with jshint & test with coverage, and add html-minifier feature

Hi,

I think it can be usefull to be able to minify html before browserify insert it in the JS code, because when you use uglify to minify the JS code, the HTML is always well formatted in the minified file. So I added html-minifier to package dependencies. 

You can specify which extensions will be minified, and you can override the default html-minifier options. This is how it works:

```javascript
stringify({
  extensions: [...]
  minify: true,
  minifier: {
    extensions: [...]
    options: {
        // html-minifier options
    }
  }
})
```

I add some gulp tasks too:

```gulp lint``` JSHint validation
```gulp test``` Run the mocha tests
```gulp test --coverage``` tests and show code coverage in the terminal
```gulp test --coverage --browser``` tests and show code coverage in the default browser

Of course I added some unit tests and I updated the README file.