Made changes asked in review by timgraham

* Corrected the documentation
* Updated 1.8 release notes
* Removed intermediate variable in `has_module_permission()`
* Added uses of `has_module_permission()` in options.py
* Replaced `user.has_module_perms()` by
  `model_admin.has_module_permission()` in sites.py
* Added test for overriding `has_module_permission()`