Fix #4 and also many other changes.

1. Now the app has an Icon.
2. There is detailed instruction about how to run the app.
3. Internal and External monitors are renamed to Primary and Secondary.
4. Single quotation is used in every place of brightness.py .
Use Internal and External in GUI
Merge pull request #6 from apandada1/master

Pull request from apandada1, which fixes #4. Also,

1. Now the app has an Icon.
2. There is detailed instruction about how to run the app.
3. Internal and External monitors are renamed to Primary and Secondary.
4. Single quotation is used in every place of brightness.py .