zip and gzip method which works like python's itertools.zip and izip.

[HennyH]

This branch adds the useful zip and gzip methods which are functionally equivalent of python's itertools libraries' zip and izip respectively.

For example:

> l = { {'a','b'}, {'c','d'} }
> r = { {1, 2}, {3, 4} }
> zipped = __.zip(l, r)

Results in zipped having:

{
    { 
        { "a", "b" }, { 1, 2 } 
    }, 
    { 
        { "c", "d" }, { 3, 4 } 
    } 
}

And then gzip:

> for li, ri in __.gzip(l, r) do print("Recieved", inspect(li), " , ", inspect(ri)) end
Recieved        { "a", "b" }     ,      { 1, 2 }
Recieved        { "c", "d" }     ,      { 3, 4 }

I have also added a line to spec_helper so it picks up the underscore.lua in the lib folder.