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.