File names and links fixed

Hey,

these are the changes I made overall so far. They are "fixing" the following issues we had with our blackboard export:

- The filenames in moodle contained the blackboard resource_id (xid_@@@@@@_1), that didn't look very nice, so I am cleanng that up.
- The links to the files were incorrect. They path and filename was a bit off and thus the file could not be accessed. I did more or less the same changes as above for cleaning up the filename and now they work fine.
- In the quiz module, it is possible to embed images in questions. From what I figured, these links remained entirely unconverted and could not be loaded. To fix this, I replaced the blackboard link with the moodle path of the file directly in the .dat file. However, the course id in the moodle system is required to link to the file so as a workaround, I added an optional parameter that you can pass to reteach to send the course id in case the course has quizzes to be converted.

Unfortunately, I do not have access to our blackboard installation (just the exports) so I cannot really tell why it wasn't working from the start. Check out the changes when you have time and tell me what you think.

Thanks!!