Made corrections to the name of the IPP operation (related to #65)
Merge pull request #69 from sbellem/create-job-subscription-corrections

Made corrections to the name of the IPP operation (related to #65)
Adapted create printer subscription request to conform to RFC 3996 - fixes #68
Removed unnecessary explicit display of notify-subscription-id attribute
Removed problematic defaults
Merged common subscription operation attributes into one base class
Changed some test values
Merge pull request #71 from sbellem/subscription-operations-corrections

Subscription operations corrections
Implemented Get-Notifications operation - closes #70
Merge pull request #73 from sbellem/get-notifications

Implemented Get-Notifications operation - closes #70
Fixed #72
Merge pull request #75 from sbellem/issue-72

Fixed #72