Organize imports

The main goal of this reorganization is to avoid wildcard imports and
"transitive" imports. After all, explicit is better then implicit [1]
and makes it much easier to understand where things are coming from.

All tests from [2] passed.


1. Wildcard imports:

from catmaid.models import *
from catmaid.control.authentication import *
from catmaid.control.common import *

can, for example in the case of log.py, be replaced with:

from catmaid.models import UserRole, Log
from catmaid.control.authentication import requires_user_role 


2. Transitive imports are confusing. See this example: 

from catmaid.control.authentication import requires_user_role, UserRole

This is not wrong, but relies on the authentication module importing
UserRole [3]. UserRole is a model from catmaid.models, so that's where
it should be imported from:

from catmaid.control.authentication import requires_user_role
from catmaid.models import UserRole



[1] http://legacy.python.org/dev/peps/pep-0020/
[2] https://github.com/acardona/CATMAID/commit/edadf8c08bbe
[3] http://stackoverflow.com/questions/18722531
Fix whitespace/indentation errors

no changes to the code other than changing indentation from 2 or 3
spaces to 4.
Label: fix typo
Ontology: fix typo

relid -> rel_id
Merge pull request #800 from fzadow/fix-imports

Fix imports