Support for recursive tables

[o-lim]

Adds support for copying and comparing recursive tables, and disables deep copies for spies.

• Allows spies to be passed as arguments to other spies
• Allows comparing tables with recursive references to itself

Fixes issues #54, and olivine-labs/busted#425

[mpeterv]

The fix for recursive table comparison is incorrect. E.g. this assertion passes:

local luassert = require "luassert"
local t1 = {}
local t2 = {}
local a = {}
local b = {}
local c = {}
local d = {}
t1.k1 = a
t2.k1 = b
a.k1 = c
b.k1 = d
c.k2 = a
d.k2 = d
luassert.is_table(t1.k1.k1.k2.k1)
luassert.is_nil(t2.k1.k1.k2.k1)
luassert.are_same(t1, t2)