created separate map_spec.js and fixed #33

I actually don't like that I needed to put special logic in
MappedIndexedSequence#get and MappedArrayWrapper#get. I will
think about a better approach to this (and also about
refactoring these specs).
renamed spec_common.js -> spec_helper.js
moved flatten specs into their own file
switch from 'window' to 'this'

This way you can use Lazy.js in a web worker without the 'this.window =
this' hack.