Added all possible exceptions to the examples of the at functions.

This test case relied on logics that have been replaced by CMake with
#461. This change enables compilation and execution of the test suite
without exceptions by adding an after_success task.

Implemented std::less<value_t> to allow using value_t as std::map key
in MSVC.

MSVC does not define __cpp_exceptions, but seems to use _CPPUNWIND when
exception support is switched on, see
https://msdn.microsoft.com/en-us/library/b0084kay.aspx.

I used __EXCEPTIONS to detect whether exceptions are supported.
Apparently, this is a macro that is only used by libstdc++
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64276). It’s much cleaner
to use __cpp_exceptions as it is in the standard since C++98.

Note that compiling the unit-tests with “-fno-exceptions” still does
not work, because Catch uses throw internally. However, the library’s
exceptions can be switched off by defining JSON_NOEXCEPTION.

Ran “make pretty” and added a note to the README file.

Fix -Weffc++ warnings (GNU 6.3.1)

MSVC needs this overload to compile code containing a std::map that
uses nlohmann::detail::operator as key.

Adds a to_json function for std::vector<bool> to allow implicit
conversion from bit vectors to basic_json.

The original test case relied on an invalidated iterator. This error
did not occur before, but only with GCC with -D_GLIBCXX_DEBUG. This
commit fixes the test case. The library is unaffected by this change.

Replacing references to std exceptions with user-defined exceptions.
Also changed some examples to the new exceptions.

Tests for parse_error.109 were missing.

Also moved all exception classes into the detail namespace and
introduced them via “using” into basic_json.