+[feature] get sync if no callback
+ put and del can be sync now.
+ batch can sync if no callback
* [bug] memory leak fixed
+ approximateSize sync if no callback
+ readOptions,writeOptions to Database for sync
+ open sync if no callback
+ benchmark sync
+ {asBuffer:false} benchmark test
+ openSync, putSync, getSync, delSync, batchSync
* minor bug fixed
* optimal iterator performance
+ nextSync, endSync on iterator
* some memleak fixed
* optimize iterator performance
+ Iterator.nextSync, Iterator.EndSync
+ ApproximateSizeSync
* forget to publish openSync to js
+ using the new snowyu/abstract-leveldown#feature/sync to test
* update readme