Pull request with the a PRE-RELEASE for April-ANN v0.3.1-beta

Please, look carefully all the changes, in order to verify them. This release incorporates interest_points merge, to remove warnings during compilation. When this pull-request was accepted, the PRE-RELEASE version will be available at the master. After this PRE-RELEASE was tested successfully, it will become the RELEASE v0.3.1-beta.