Change WePay::$all_scopes to WePay::getAllScopes()

Having a public static variable means that one part of the application may inject anything into this variable and another part of the application will use it. Change it to a method and leave  the original variable deprecated (for backwards compatibility).
Change mentions of WePay::$all_scopes to WePay::getAllScopes()
Merge pull request #7 from vrana/patch-3

Change WePay::$all_scopes to WePay::getAllScopes()