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.

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

minor changes, but the core idea remains the same