Add fromIterator helper for Enumerator

Enumerator.fromIterator is a function that creates a enumerator from an
Iterator.
Change Enumerator.fromIterator to Enumerator.fromTraversableOnce

With respect to the review of @sadache, @julienrf and @mandubian
Avoid using closure, use scoped variable instead.
Rename fromIterator to enumerate

As @sadache suggested renamed o enumerate. Adding a parameter to the
apply is not without breaking the current API though.
Merge pull request #633 from bobylito/master

Enumerator.fromIterator