Remove auto-ids on non-core constructs

Fixes #174 

Removed auto-generated IDs on:
- Identity
- Kill Chain
- Kill Chain Phase
- Test Mechanism
- Attack Pattern
- Exploit
- Infrastructure
- Malware Instance

This leaves only the items listed in [Assigning IDs](http://stixproject.github.io/documentation/suggested-practices/) as those that are auto-assigned IDs (assuming CybOX is kind enough to provide IDs for its objects.)