notes

** TODO:
1. Add support for scoring model choice, such as sum up the field score or just use the max field score.
2. Code review, make it more efficient.
3. search configuration format verification, check that user's configuration is correct and legal.
4. change addFieldLength() in index.js when building index to addDoc() function call.
5. removeDoc() also need to remove relevant document field length info.
6. add type configuration for search, such as "most_type", "best_type"
7. add test cases for issue fix #8 [#8](https://github.com/weixsong/elasticlunr.js/issues/8#issuecomment-193266856), boolean "AND" error.
8. add test cases for documents update issue. [#12](https://github.com/weixsong/elasticlunr.js/pull/12), thanks [deerawan](https://github.com/deerawan).

* add PriorityQueue class, defaultly return only top 100 results, that could reduce memory used.
* support query config: from to, store query results and don't do repeated query.
* only search top k results, need PriorityQueue support.