Added functions based on PyTrie

Hey,

I've added `iter_prefix_values`, `prefix_values`, and `longest_prefix_value ` to BaseTrie. Also implemented the same methods in Trie subclass, using the values to index into `_values` attribute. This should close [issue 8](https://github.com/kmike/datrie/issues/8).

I tried to follow your structure as closely as possible - single underscore for cdef functions to then be called in Trie subclass, etc.

Also added tests and new entries in speed.py.

Really not that big of a deal but I noticed BaseTrie uses a signed int so it actually works with -2147483648 <= x <= 2147483647. I changed it in the README and docstring.

Thanks,
Jared