Some code refactoring to add a GetAllTracks() method in Album.cs

I needed to get all the tracks from a specified album Id so I created the "GetAllTracks" public method as well as the "GetTracksByAlbumId" private one (I used a public and private method in order to stick to the same practices than the rest of the code).

I had to move part of the URL in GetUrl() to move the "search.php" part to the "By*****" methods since not all requests will go to that page.

This is my first pull request ever, I hope it's got all that is needed to go ahead!