ammed user.get method to accept userid parameter
document the difference between userid and username, and give
examples of circumstances where they might differ

fix typo in parameter designation
document that the method may raise UserNotFoundError
ensure that the username passed is treated exclusively as username, not
aliased to userid

raise UserNotFoundError if the provided username cannot get a user

document that this error may be raised
treat username as username only, do not alias to userid

raise UserNotFoundError if user cannot be found by username

document that this error may be raised
ensure that the non-existent user does not raise the wrong error
fix test to catch the correct problem

fix test to check for the right exception type
add test that removing a user from a group raises the appropriate error in
the case that the user does not exist
add changelog entry describing the potentially backwards-incompatible conditions around this bug and the fix
flake8 and typos
style fixes from comments, thanks @zupo
style fix per @zupo
Make a helper method which asserts that the authenticated user has a
different userid and username.  This method is used in tests which
demonstrate taking action by username vs. userid to ensure that should
this assumption no longer be valid, we can learn of the fact.
style fix per @zupo
fix incorrect username/userid usage
style fixup per @zupo
Merge pull request #139 from plone/issue_112

Amend user.get method to accept a userid parameter.