Add API versioning support, including tests
Run all tests at all API versions
Handle case where tag count is returned as string (as was the case in APIv1)
next/previous fields are lists in APIv2, but single photo ids in APIv1.
Update README with API versioning documentation
Merge branch 'master' into api_versioning

Conflicts:
	tests/test_base.py
Merge branch 'master' into api_versioning

Conflicts:
	tests/test_tags.py
Error in future API test now correctly raises a 404 exception.
Merge pull request #29 from sneakypete81/api_versioning

API versioning support