Split out some code into the create_unique_index method to prepare for merging with a codebase that has summitborar's changes. (Plus, it was a good idea he/she had.)
Renamed create_unique_index to create_indexed_vocab because this better captures the method's purpose.

Also updated method docstrings to mention that create_unique_index is called from build_vocab and is a prerequisite to create_binary_tree