Change syntax to be Python 2.6 compatible
Add support for user impersonation

http://www.redmine.org/projects/redmine/wiki/Rest_api#User-Impersonation
Add user impersonation comments
Merge pull request #25 from knabar/user-impersonation-pr

Add support for Redmine user impersonation
Merge pull request #24 from knabar/python-2.6-fix

Change syntax to be Python 2.6 compatible