Changed DominatorTree to have getHeads

#### The Bug:

DominatorTree's used to have getHead() as the method to get the head of
the tree. Unfortunately, if it was a post-dominator tree and you wanted
to compute the post-dominator frontier this caused problems for
functions with multiple distiguished exits (eg. more than one head in
the post-dom tree). Basically, what would happen is the frontier would
miss elements for certain graph constructions.

#### The (Proposed) Fix:

I have fixed this problem by removing the getHead() method and replacing
it with a getHeads() method. I also cleaned up a lot of whitespace
issues in these files.

This patch may not be the best way to go about fixing this issue. Let me
know what you think would be the right way to go about this. Since this
is my first contribution to soot I am not sure what the expectations
are.