managing users groups enforce permissions checks.
User needs at least a read permissions on usergroup to be able
to assign it somewhere.

UserGroup on UserGroup permissions implementation.
- user group can have another usergroup assigned
- it's impossible to assign usergroup on the same usergroup

implemented usergroup permissions checks.
- each user who has now explicit or inherited permissions can create usersgrous
- user is automatically an admin of that usergroup, and can manage it

- Manage User’s Groups: create, delete, rename, add/remove users inside.
by user group admin.
In this case, a user's group can be owned by several people thru an owner user's group.

Some refactoring of naming, permission handling logic.
- remove some code duplicity as well as inconsistent naming

follow Python conventions for boolean values

True and False might be singletons and the "default" values for "boolean"
expressions, but "all" values in Python has a boolean value and should be
evaluated as such. Checking with 'is True' and 'is False' is thus confusing,
error prone and unnessarily complex.

If we anywhere rely and nullable boolean fields from the database layer and
don't want the null value to be treated as False then we should check
explicitly for null with 'is None'.

further cleanup of UsersGroup

Renaming some names ... but trying not to change API or database scheme.

"Users groups" is grammatically incorrect English - rename to "user groups"

- and same story with "repositories groups"

This will only change the user facing texts. Names of files, modules, functions
and variables should be changed too.

RhodeCode now has a option to explicitly set forking permissions. ref #508
- changed the way permissons on users groups behave. Now explicit set on user
is more important than permission set on users group

New repo model create function
- fixed some sessions calls

API changes
- added removed_user_from_users_group function
- fixed errors on wrong username
- normalized add/remove members from groups functions
- docs updates

#227 Initial version of repository groups permissions system
- implemented none/read/write/admin permissions for groups
- wrote more tests for permissions, and new permissions groups
- a lot of code garden, splitted logic into proper models
- permissions on groups doesn't propagate yet to repositories
- deprecated some methods on api for managing permissions on
repositories for users, and users groups

commit less models
- models don't do any commits(with few exceptions)
- all db transactions should be handled by higher level modules like controllers, celery tasks

Fixed permissions for users groups, group can have create repo permission now.
Some code refactor + pep8ify

#56 implemented users groups editing,
fixed sqlalchemy relation function into relationship (according to 0.6)

#56 fixed found bugs, implemented adding of new group + forms+validators
fixed db schema naming