Add uniq db index on project_id+iid
Add iids changelog entry
Migrate invalid rows with missing iids or duplicates
Add rake setup alias for gitlab:setup
Add doc file for dev rake tasks
Merge branch 'index_on_iid' into 'master'

 Add uniq db index on project_id+iid

Because:
* its faster
* it solves problem with repeating iid's in one project
Merge branch 'rake_setup' into 'master'

rake setup

Add alias `rake setup` to `rake gitlab:setup`
Mention in developer docs
Merge branch 'master' of https://github.com/gitlabhq/gitlabhq