Added rich comparisons and __iter__ to _Trie

Closes issue #16.
I guess for equal weights the ordering produced `WEIGHT_ORDER` is still lexicographic. The only occurrence of `WEIGHT_ORDER` in `louds-trie.cc` is at [`louds-trie.cc:374`](https://code.google.com/p/marisa-trie/source/browse/trunk/lib/marisa/grimoire/trie/louds-trie.cc#374). The line does a stable sort on a vector of weighted ranges, which implies that for equal weights the order doesn't change.

Anyway, I've modified the code to use `Agent` and return `False` on node order mismatch. Where's the best place to document the node order behaviour?