Fixing all the failing tests and removing Cache

**Cache removed**
When testing with the tests provided by PyFilesystem I discovered that `DropboxCache` was not working correct and in order to fix all the failing tests this would have to be completly rewritten.
Now PyFilesystem does have the concept of a CacheFS which actually caches metadata information like weather a file exists or not: Instead of implementing an own cache in the `DropboxClient` we could simply wrap the `DropboxFS` in a `CacheFS` to allow speedup. This will also result in correct behaviour as the `CacheFS` is well tested.

For real performance improvement by the `CacheFS` the `DropboxFS?  implementation has to use it's methods (like `exists`) directly and not encapsulate that functionality in the `DropboxClient`, not only is this a cleaner and self contained approach, it is also cacheable.
This has still to be done.

**Fixed failing tests**
This mostly concerns errors that were not raised. But there were also a few features that were not supported by directory and file manipulation operations.