Removed the call to require 'activationhelper' from three controllers
Refactored and simplified import_users method
Small cleanup of variables in ImportUsersController
Refactored error and validation from #import to #valid_input_file? and added before_filter
merged austin's changes for #372
Merge pull request #376 from YaleSTC/375_remove_require_activationhelper_bugfix

Removed the call to require 'activationhelper' from three controllers
fixed merge conflicts