Add a colorpicker-is-open attribute for controlling the visibility of the popover

Continuing on from https://github.com/buberdds/angular-bootstrap-colorpicker/issues/66 here's my implementation. I tried to include a unit test but I couldn't get your tests to run. It kept giving me the error:
```
TypeError: Cannot read property '$injector' of undefined
	    at Object.workFn (/Users/mattlewis/Sites/angular-bootstrap-colorpicker/test/libs/angular-mocks.js:1698:15)
TypeError: Cannot read property '$modules' of undefined
	    at Object.workFn (/Users/mattlewis/Sites/angular-bootstrap-colorpicker/test/libs/angular-mocks.js:1769:25)
TypeError: Cannot read property 'setKnob' of undefined
	    at Object.<anonymous> (/Users/mattlewis/Sites/angular-bootstrap-colorpicker/test/unit/colorpickerSpec.js:166:13)
TypeError: Cannot read property '$injector' of undefined
	    at Object.<anonymous> (/Users/mattlewis/Sites/angular-bootstrap-colorpicker/test/libs/angular-mocks.js:1641:24)"
```

Any ideas?