Use released version of Level

[jayvdb]

Currently depending on "level-js": "philschatz/level.js#master" , which has one extra commit from a branch 108 commits behind master of https://github.com/Level/level-js

The latest released version of level-js is quite different, and likely includes the needed configurability which that extra commit provided.

[jayvdb]

Also the related "levelidb": "^0.2.6" causes a dependency on git repo https://github.com/Raynos/IDBWrapper.git@7958b742d7d41432bbf1f897ee1507edc6bff64e

[li-boxuan]

Seems the needed configurability is not provided by release version of level-js.

But levelidb can be safely removed I guess, I don't see any dependency on it.