It was a good idea to implement a const version of operator[] it in the
first place. I was a pity that this implementation was flawed. It was a
mistake to remove the const version completely. This commit
re-introduces the const version. My apologies for all the inconvenience.

added value() function to get object value at given key or a default
value if key does not exist

In a string the first character following a surrogate pair is skipped by the
lexer, but the rest of the string is parsed as usual.

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

It doesn't really make sense to stream an iterator.
Besides, this was causing trouble in MSVC in Debug mode (see #125)

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