Replace the disable attribute of uid fields with the readonly attribute.

Controls that are disabled cannot be successful and the values
contained in the disabled controls will never be submitted.

Since the uids fields are disabled and  are not submitted,
the uid_to_issue_id hash created in Import.import_tasks method
are not constructed correctly.
Merge pull request #18 from buri17/fix-disabled_uid_field

Replace the disable attribute of uid fields with the readonly attribute.