Map#map and Map#filter

Added proposal for [Map.prototype.map](https://gist.github.com/DmitrySoshnikov/a218700746b2d7a7d2c8), and [Map.prototype.filter](https://gist.github.com/DmitrySoshnikov/82b46f4674acee5cedc2).
Merge pull request #13 from DmitrySoshnikov/master

Map#map and Map#filter