Issue title: Type or namespace ColorChangedEvent/HSVChanged Event could not be found
 Issue body: Hi.  Using Unity 2020.2.0b2.3094:

Assets/HSVPicker/UI/ColorPicker.cs(18,16): error CS0246: The type or namespace name 'ColorChangedEvent' could not be found (are you missing a using directive or an assembly reference)?

Assets/HSVPicker/UI/ColorPicker.cs(19,16): error CS0246: The type or namespace name 'HSVChangedEvent' could not be found (are you missing a using directive or an assembly reference)?

Thanks!
<img width="1269" alt="HSVPickerError" src="https://user-images.githubusercontent.com/75796675/101788386-0797e080-3ace-11eb-9efa-213abad25607.png">

 Comments: 
 Comment 0: Version : Unity 2020.2.2f1
Unity Package Version : Color Picker 3.0.1

1. Import Package
2. Edit "HSVPicker.asmdef"
- Add Use GUIDs > UnityUIExtensions
- ![image](https://user-images.githubusercontent.com/3183770/106562602-dc7dba80-656d-11eb-9415-3d5effe2c3cb.png)
3. Each Error Script To Add "using UnityEngine.UI.Extensions.ColorPicker;"
- ![image](https://user-images.githubusercontent.com/3183770/106562636-eb646d00-656d-11eb-969d-824908373da1.png)

{'source': 'GithubIssues', 'repo_name_with_owner': 'judah4/HSV-Color-Picker-Unity', 'issue_id': 761311894, 'repo_stars': 500.0}