fix handling of test mechs

This allows test mechanisms to be parsed under all circumstances, even if a colon is missing from the type. 
Caused by an indentation mix-up

Repro:

```
#!/usr/bin/env python

from stix.core import STIXPackage
from stix.extensions.test_mechanism.generic_test_mechanism import GenericTestMechanism

	stix_package = STIXPackage.from_xml('bro.xml')

	for indicator in stix_package.indicators:
		print "== INDICATOR =="
		print "Title: " + str(indicator.title);
		print "Description: " + str(indicator.description);


        for tm in indicator.test_mechanisms:
		print tm.description;


```
> 
File "/home/ubuntu/workspace/stix/bindings/indicator.py", line 691, in buildChildren
UnboundLocalError: local variable 'obj_' referenced before assignment