As discovered by @felipegb94, there are missing overloads to the
`GenericValue::AddMember<T>` template function, taking an explicit
`GenericValue&` as a name and accepting arbitrary primitive values.

This patch adds the missing overloads. The `StringRefType` overload
is needed to disambiguate the addition of a string literal as
value.

Some tests are added to `TEST(Value, Object)` in `valuetest.cpp`.

The tests state the current traits of types Document, Value and
StringBuffer. There are slight differences between them. It seems like a
good idea to extend this approach across more types, and to review the
expected traits across the board.

See https://code.google.com/p/googletest/wiki/FAQ#Why_should_not_test_case_names_and_test_names_contain_underscore

Prepare equalto_operator tests to test comparisons between
 * GenericValue and GenericDocument
 * GenericValue with different SourceAllocator types

Both combinations currently fail due to ambiguities with the
templated operators on several compilers.

This reverts commit 9ab2621e.

As std::vector::erase() overloads added in C++11