Add initial versions of zip and merge

Refactor overall architecture to allow combinators to be coded
separately from core Stream type.  Move Stream to lib. Add new most.js
main module that aggregates Stream type and combinators.

Modularize combinators and their unit tests.  Add merge and zip.
Merge pull request #15 from cujojs/reorg-combinators-add-merge-zip

Refactor combinator architecture