Fix term names according to specification.

Since now the images, flavor and secgroups categories and links are
based on the ID, we have to adapt all the code to reflect this change.

Closes #38
Ensure that the appended categories does in fact exist.

Closes #43
Merge branch 'bug/38' into stable/folsom

Conflicts:
	occi_os_api/registry.py
	occi_os_api/wsgi.py