JSONCONS_ASSERT uses std::cerr, so include <iostream>.

Easily seen by adding jsoncons.hpp as the first include in a bare-bones cpp.
Silence compiler warnings for unused variables

As seen by Clang with -Wextra.
This is probably not all of them, but it includes all of the cases that my code
ran into.
Merge pull request #20 from theuni/compiler-warnings

Silence compiler warnings for unused variables
Docs
Merge pull request #19 from theuni/missing-header

JSONCONS_ASSERT uses std::cerr, so include <iostream>.
acknowledgements
Readme
Fixed issue with operator[i] on json values of type object_t
Merge branch 'dev' of https://github.com/danielaparker/jsoncons into dev
json constructors that take string, double, etc. are now explicit.
get with default now allows extended types as default value
Updates to docs, simplify assignments in tests and samples
make_multi_array renamed to make_array, update to docs