Add `file_name` as valid argument  to Account.get_files

When trying to use `account.get_files(file_name='*.json')`, the following error is logged:

    WARNING:root:Invalid arguments found: file_name

The `file_name` argument is ignored and all files found are returned. This is not expected behavior!

Since [version 2.0 of accounts/files API](http://context.io/docs/2.0/accounts/files) supports this argument, adding this fix causes the client to behave as expected