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

remove invalid parameter '-stdlib=libc++' in CMakeLists.txt

Replace sprintf with hex function, this fixes #149

Fix character skipping after a surrogate pair

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.

removed stream operator for iterator, resolution for #125

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.

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

exception-safe object creation, fixes #118