add another test for file_line_iterator
added sentence_iterator and sentence type

`sentence` is just a typedef of how i propose we represent these
sentences:

```c++
std::vector<std::pari</* word */ std::string, /* POS */ std::string> >
```

`sentence_iterator`, as the name implies, iterates over all of those.
since it's an iterator it should be able to cover a lot of sentences w/o
eating up any more memory than necessary.

GOOD LORD IS IT SLOW. like 3 seconds just to iterate through all of the
files. i think my python implementation is faster. c++ makes me feel
like half a programmer. i think the problem is it does a lot of string
copying, but i'm not strong enough in c++ to really know how to fix that
right now anyway.

also it incorrectly handles sentences with quotes after the last period.
i don't think this is worth fixing right now, but i'll file an issue.

TODO: check for memory leaks

pertains to #3