Implemented 'PersistentMap.fromPairs' factory constructor to allow creating a PersistentMap from an instance of Iterable<Pair>>

That allowed creation of 'strictMap' and 'strictWhere' versions of 'map' and 'where' that return PersistentMap instead of Iterable<Pair>.
Created [](K key) operator that calls lookup and made PersistenceMap a mixin.
Changed signature of operator [](key) to return V instead of Option<V> and moved it's implementation to PersistentMapBase.

Updated docs in PersistentMap.fromPairs constructor.
Merge pull request #2 from rafael-st-rj/master

Implemented 'PersistentMap.fromPairs' factory constructor to allow creating a PersistentMap from an instance of Iterable<Pair>>.