deposit: save field after import fix

* Validates each field modified after import instead of saving
  the whole form. (Closes #80)

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
deposit: contributors rename to authors

* As we are triggering a dataSaveField event to save each imported
  field, the name of the field should match the name in
  forms.py.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
deposit: plupload allow PDF only

* Adds filters to plupload in order to only allow uploading PDFs. This
  filter can be extended in the future to add more mime types as needed.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>