add support for Scala maps

Maps are translated to javascript objects. The higher-order functions
`foreach`, `mapValues` and `filter` are encoded using the `Object.keys`
method the `forEach` iterator on arrays
small fixes and improvments
small fixes

allow for nicer use in for-comprehensions for `foreach` and `filter`
operators. Some test added on filter followed by foreach
Merge pull request #6 from gnieh/maps

Add support for Scala maps