Jira: Added cluster methods (#586)
Jira: Added cluster methods (#586)

* Added parameter for user() method:
 - get user by key

Now you need to use only named parameters.
For example, user(username='john') or user(key='john.d').
You can use only one parameter: username or key.

* Jira8 set deleted as unnecessary. All improvements will implementing in just Jira set.

* Added cluster methods.
AstroTech/atlassian-python-api,MattAgile/atlassian-python-api,AstroTech/atlassian-python-api