Add a jatos ID to the osweb result set

[dschreij]

Potentially related to: #16

Currently, the metadata set by Jatos is no longer stored in osweb's own data. It is set as a context field in the results of a participant, or can be downloaded through the export metadata button in Jatos. The problem is that neither of these is added to the exported result set if it is saved through this extension.
So either we need to make sure that the context data is included in the exported data by this extension (as proposed in #16), or add an identifying variable (e.g. jatos.ComponentResultID) to osweb's data, enabling users to associate downloaded metadata to the corresponding osweb results themselves.

Also see https://forum.cogsci.nl/discussion/comment/21084 for a discussion that prompted the creation of this issue.