Have Tokenizers return a Token object

[matt-gardner]

This will let us get rid of the nasty offset return value, because it will just be a field on the Token, and it will let us include POS tags, for POS tag embeddings.

It's probably easiest to just return spacy's token representation directly, rather than trying to roll our own, and have other word splitters mimic spacy's API. Or we could just have them crash; not sure we really need the other word splitters at this point - we could just simplify things a lot by putting spacy directly into WordTokenizer. Anybody have any thoughts on that?