Issue170 swid measurement controller

- Added the software-id to the tag model. Therefore the following databse change is needed (additional field software_id):
```
Drop table "swid_tags";
CREATE TABLE "swid_tags" (
    "id" integer NOT NULL PRIMARY KEY,
    "package_name" varchar(255) NOT NULL,
    "version" varchar(32) NOT NULL,
    "unique_id" varchar(255) NOT NULL,
    "swid_xml" text NOT NULL,
    "software_id" varchar(255) NOT NULL
);
```

- The Controller can be tested with: 
````
curl -i -X POST http://localhost:8000/api/sessions/2/swid_measurement/ -H "Content-Type: application/json" -d '["regid.2004-03.org.strongswan_debian_7.4-x86_64-strongswan-4.5.2-1.5+deb7u3"]' -u root:root
```
(The "regid.2004-03.org.strongswan_debian_7.4-x86_64-strongswan-4.5.2-1.5+deb7u3" software-id exists in our repo db.)