dropbox _dir.r & _search.r & _delete.r api url changes.

Hi,

I was trying to run the rDrop for a specific folder located in the Apps directory of my Dropbox (i.e, I didn't give rDrop a complete control over my Dropbox). May be this is the reason why I've encountered the following errors while trying to display the files in the directories or if I'd want to delete the files. Therefore, I've made these small changes which helped in resolving my issue. If its okay, you can test and accept the commit.

```R
> dropbox_dir(dropbox_credentials)
Error in fromJSON(OAuthRequest(cred, url, list(include_deleted = deleted),  : 
  error in evaluating the argument 'content' in selecting a method for function 'fromJSON': Error: Bad Request

> dropbox_delete(dropbox_credentials, file_to_delete='x.rda')
Error in fromJSON(OAuthRequest(cred, "https://api.dropbox.com/1/fileops/delete",  : 
  error in evaluating the argument 'content' in selecting a method for function 'fromJSON': Error: Bad Request

> dropbox_delete(dropbox_credentials, file_to_delete='x.rda')
Error in fromJSON(OAuthRequest(cred, "https://api.dropbox.com/1/search/dropbox/",  : 
  error in evaluating the argument 'content' in selecting a method for function 'fromJSON': Error: Bad Request
```

Cheers,
NM