Feature/segment editor

Please, review.
This branch contains changes in the SegmentEditor plugin that enable setting the access level required to create/edit/remove segments. 
It must be set in the config.ini.php under the General values. An example has been added (commented out) in global.ini.php: segment_editor_required_access = "admin".
The possible values for this parameter are: "view", "admin", "superadmin" and are compared to the user's access level to a given siteid.
If there's no siteid given and the user is not a superuser then the user has no access to segment editor, and if the segment is accessible for any site then the user needs the required access level for at least one site. 
User always can view segments from the segment selector (given he has some view access).