GenericDocument::Accept: deep-copy strings, if needed

Instead of always just shallowly referencing the potentially allocated
strings when calling the Handler::String function, request a copy in
case the string has been allocated from an Allocator before.

This is necessary to avoid double free()s of the string memory,
especially when using the Handler to create a deep copy of a Value.
GenericValue: add copy constructor and CopyFrom

To allow deep copying from an existing GenericValue, an
explicit "copy constructor" (with required Allocator param)
and an "CopyFrom" assignment function are added.

  Document d; Document::AllocatorType& a = d.GetAllocator();
  Value v1("foo");
  // Value v2(v1); // not allowed

  Value v2(v1,a);                             // make a copy
  RAPIDJSON_ASSERT(v1.IsString());            // v1 untouched
  d.SetArray().PushBack(v1,a).PushBack(v2,a);
  RAPIDJSON_ASSERT(v1.Empty() && v2.Empty());

  v2.CopyFrom(d,a);                           // copy whole document
  RAPIDJSON_ASSERT(d.IsArray() && d.Size());  // d untouched
  v1.SetObject().AddMember( "array", v2, a );
  d.PushBack(v1,a);

Additionally, the Handler implementation in GenericDocument is made
private again, restricting access to GenericReader and GenericValue.
valuetest: add deep copy unit test

This commit adds some simple tests for the deep-copying
of values, either based on the explicit constructor, or
the CopyFrom function.

It uses the CrtAllocator to test for possible double-free
errors due to insufficient copying.