Work on instructors and members controllers

Work on #create, #update, and #destroy actions in MembersController and InstructorsController.
Add a comment to members_controller

Add a note that MembersController is to be used for experiments.
Member's activity and memberships in register

Display members's activity and memberships when preparing a new transaction in register controller.
Allow to re-initialize model associations

Allow to re-initialize model associations assuming that the associated classes changed but their names stayed the same (in a new namespace/module/class).
Avoid repetitions in Accessors module

Use meta-programming to avoid repetitions in Accessors module.
Simplify MembersController with meta-programming
Simplify RegisterController
Clean up
Change filtering

Use new classes SimpleRelationFilter and FriendlyRelationFilter and a new ApplicationController private method #do_filtering, instead of including a filtering module into all models,
Work on PseudoColumns module and its use
Extend PseudoColumns to remember column names

Extend the PseudoColumns module to remember column names as strings.

Attention: the instance variable @column_names already exists in ActiveRecord.  Added variable is called @pseudo_column_names.
Format
Change sorting

Similarly to filtering, make the controller responsible for sorting.  Do not mix in Sorting module into models, remove it completely.
bundle update
Format
Rename @attributes to @attribute_names

Rename @attributes to @attribute_names and @<something>_attributes to @<something>_attribute_names.
Rename @.._associations to @.._associaiton_names
Do not use before_filter to simply find a record
Custom form builder: do not select among >15
Fix GuestEntry-MembershipPurchase association