New convenience methods

This release has the following changes:

• Add the matrix_copy method to get a numpy array copy of the embedding matrix.
• Add the vocab method to get a Vocab instance, which provides the item_to_indices method to get the indices or subword indices of a word. Vocab also provides indexing to look up the word corresponding to an index (e.g. vocab[3823]).
• Upgrade to finalfusion 0.6.