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