Add support for order=1 to TrieModel

[changq]

change it to support ngram-order=1 (tested for TrieModel)

1. don't throw when order=1
2. do some initialization only when order > 1 to avoid segfault
3. set default RecordReader::remains_ to false to pass a later conditional throw on ngram-order

[kpu]

Not the most efficient way (the unigrams will have useless trie pointers and backoff). Should really be its own implementation but Holger Schwenk wants something too. . . so sure.

[changq]

Yes, I understand that a separate implementation is more appropriate in terms of efficiency. In building a classifier, I need to use from unigram to pentagram as features. It will be wonderful to use KenLM as a unified solution to manage these features. So I've made the code change and submit this pull request for your consideration.

[lpcauch]

Hey @changq ,

How do you create a lm.binary after you create your unigram ?
I succeeded to create the unigram but can't create the lm.binary (it throws an error "Was expecting n-gram header \1-grams: but got \end\ instead Byte: 209").
I just download your git, no modification have been done so I don't know what to do :/
Any helps is welcome !