Issue 46. Added AttachmentManager class with addAttachmentToIssue()

moved issue-specific methods from RedmineManager to IssueManager.
similarly: attachment-specific methods are now in AttachmentManager

moved issue-specific tests to new IssueManagerTest class.

Moved more methods from RedmineManager to AttachmentManager.
Moved user-specific methods to new UserManager class.
similarly - with tests.
moved project-specific methods from RedmineManager to ProjectManager