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.

Started implementing exceptions for invalid iterators.

This exception occurs when a JSON pointer contains an array index that
can not be converted into a number.

These exceptions occur when JSON pointers are malformed.

These exceptions are thrown in case of parse errors in JSON patch
documents.

Added class hierarchy for user-defined exceptions (#244). Integrated
parse exceptions 101-103. Parse exceptions include the byte count of
the last read character to locate the position of the error (#301).

The constructor basic_json(std::istream&, const parser_callback_t) has
been deprecated since version 2.0.0. This commit removes it together
with its code example, deprecation macro, and test cases. The code now
also compiles with -W-deprecated-declarations.

I created a wiki page
https://github.com/nlohmann/json/wiki/Road-toward-3.0.0 to describe the
transition toward version 3.0.0. On this page, all API-breaking changes
shall be documented.

The library does not preserve the insertion order of object keys. There
are frequent requests to change the library in this aspect. The README
and the contribution guidelines now contain links to containers that
can be used to replace std::map to preserve the insertion order.