Detect correctly pointer-to-const

disabled "CopyAssignable" test for MSVC in Debug mode, see #125

The intention of the current code is to detect a pointer-to-const but instead it is detecting a const-pointer. See #134

STL iterators used by json::iterator don't pass this test in Debug mode.
The test does pass in Release mode, so I felt the best thing to do was selectively disable that test.

exception-safe object creation, fixes #118

Overload parse() to accept an rvalue reference

Use the right variable name in doc string

Fix small oversight.

Remove superfluous inefficiency

fixed typos in comments for examples

try to avoid MSVC bug (https://connect.microsoft.com/VisualStudio/feedback/details/797682/c-decltype-of-class-member-access-incompletely-implemented)