Added return None instead of raising exception on getting item for resource.
Incipient total_count support for managers
ResourceSet support for total count
Fixed retrieval for total_count
Made total_count property
Set total_count to -1 if not evaluated

Makes it easier to check if total_count has been set or not
Improved total_count
Added total_count related error
Updated exception name
Merge branch 'master' of github.com:maxtepkeev/python-redmine

Conflicts:
	redmine/resources.py
limit/offset enhancements and total_count attar