Issue relations first commit (not thoroughly tested). 4 kinds of relation are available:

Issue relations first commit (not thoroughly tested). 4 kinds of relation are available:
* relates to: do nothing special. Just to know that the 2 issues are related...
* duplicates: will close the related issue with the same status when closing the issue (not implemented yet)
* blocks: will require to close the blocking issue before closing the blocked issue (not implemented yet)
* precedes (end to start relation): start date of the related issue depends on the due date of the preceding issue (implemented). A delay can be set so that the related issue can only start n days after the end of the preceding issue. When setting dates for an issue, dates of all downstream issues are set according to these relations.

To set a relation, the 2 issues have to belong to the same project (may change in the future). So if an issue is moved to another project, all its relations are removed.
Circular dependencies are checked when creating a relation.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@506 e93f8b46-1217-0410-a6f0-8f06a7374b81