Add a require('testdouble').explain function

[searls]

One thing that's always tough is debugging dynamic test doubles, in part b/c it can be problematic for them to implement a toString function of their own, and in part because any descriptors or names need to be provided to them by the caller (since they're not typically mirroring a known type). Additionally, we'll want to expose some metadata without adding methods or properties to the test double itself.

What if we had a top-level explain function that took a test double object and returned some nice descriptors:

td = require('testdouble')
myDouble = td.create()

td.explain(myDouble)

Might return:

{
  description: "This test double has 0 stubbings and has been called 0 times.",
  callCount: 0
  calls: []
}

td.when(myDouble(8,9,10).thenReturn(11)
td.explain(myDouble)

Might return:

{
  description: 
     "This test double has 1 stubbing and has been invoked 0 times. \n"+
     "\n"+
     "Stubbings: \n" +
     "  when called with (8,9,10) then return 11 \n" 
  callCount: 0
  calls: []
}