Added project module concept.

A project module (eg. issue tracking, news, wiki,...) is a set of permissions that can enabled/disabled at project level.
For each project, modules can be enabled on the project settings view ('Modules' tab).
This requires a specific permission: 'Select project modules' (if this permission is turned off, only Redmine administrators can choose which modules a project uses).

When applying this migration, all modules are enabled for all existing projects.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@725 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/fixtures/enabled_modules.yml

@@ -0,0 +1,33 @@
+--- 
+enabled_modules_001: 
+  name: issue_tracking
+  project_id: 1
+  id: 1
+enabled_modules_002: 
+  name: time_tracking
+  project_id: 1
+  id: 2
+enabled_modules_003: 
+  name: news
+  project_id: 1
+  id: 3
+enabled_modules_004: 
+  name: documents
+  project_id: 1
+  id: 4
+enabled_modules_005: 
+  name: files
+  project_id: 1
+  id: 5
+enabled_modules_006: 
+  name: wiki
+  project_id: 1
+  id: 6
+enabled_modules_007: 
+  name: repository
+  project_id: 1
+  id: 7
+enabled_modules_008: 
+  name: boards
+  project_id: 1
+  id: 8