Add Major.Minor release date to compiled assembly

The same way we currently use https://github.com/Particular/Timestamp to add a special attribute to an assembly.

The date needs to be:
- The **first** release date of a Major.Minor

Examples:
If we release v1.0.0 on 10 May 2014 then the Timestamp is 10 May 2014
If we release v1.0.1 on 12 May 2014 then the Timestamp is 10 May 2014  (same as v1.0.0)
If we release v1.0.1+x on 15 May 2014 then the Timestamp is 10 May 2014  (same as v1.0.0)

If we release v1.1.0 on 20 May 2014 then the Timestamp is 20 May 2014