Mongoskin should not truncate ES6 Promises returned from mongodb-native calls

[lbeschastny]

mongodb-native has full ES6 Promises support since 2.0.36 2015-07-07. Shouldn't mongosking support Promises too?

To do so you'll have to rewrite SkinClass._bindMethod wrapper,

If you'll approve my proposal, I may try to implement it myself and send you a PR.