Exclude option for ImportPathRegistry

* Adds option to specify a list of import paths to exclude
  from an ImportPathRegistry.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>`
Merge pull request #14 from lnielsen-cern/import-path-exclude-option

Exclude option for ImportPathRegistry

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>