XNAT assessment support

The changes add support for assessments. You can browse them and download their resources.
Other changes:
  - tooltips in the tree view
  - scans appear with their scan type instead of their label.
  - exception is caught for invalid URLs. Only for experiments at the moment, it should be handled at another level later.
No, I do not think, it's necessary. On my Mac it compiles with size_t. But I have to ask Nico because he uses Mavericks and different compiler. I will update the pull request if it's not needed.