Fix: Drilling the down the folder hierarchy / New delegate methods to handle file selection

I have noticed that wrong subpaths were generated when the user tries to drill down in the folder hierarchy. Fixed it by using `stringByAppendingPathComponent:`

Also added few new delegate methods to handle file selection explicitly. These are optional and the file is downloaded as usual when the delegate does not implement them. But when implemented they give more control to the delegate about what to do with the selected file. 

In our case we wanted to obtain shortened share links. Also added a method to obtain share links. 

I also think it would be better to bring the delegate methods to more in-line with Apple delegate methods were the first argument is always the object calling the delegate method, like: `- (void)dropboxBrowser:(DropboxBrowserViewController*)browser selectedFile:(DBMetadata*)file;` instead of `- (void)dropboxBrowserSelectedFile:(DBMetadata*)file;`