* Added new API Stats() and Prune()
* Improved Merge() logic to also prune old key/values and actually reclaim disk space
* Added backward compat for the old Merge() function
* Refactor indexing of keys to items (hints)
* Remove redundant TestOpenMerge
* Add unit test for Stats()
* Improve TestMerge()
* Fix issue(windows env):
1. Run my program firstly, it will create my.db folder normaly.
2. Close my program and restart it again, I will receive an error.
* Fix coding format issue
* Change the last fixing with @prologic suggestion.
