Better method for finding the tag index and map magic.

The old method works only if the tag array comes right after the map
index. This will always work if the map is a full version or Halo CE
map.
Find the main scenario tag

Halo supports child scenario tags. The baseId is actually the tag
identity of the scenario tag.
Merge pull request #2 from Halogen002/master

Better methods for finding tag index, map magic, and scenario tag.