Renamed ffi/libcpr.rb to ffi/ruby/libcpr.rb.
Ensured that ffi/ruby/libcpr.rb has the executable bit set.
Defined the cpr_error_callback global variable.
Corrected all local-header #include directives in source files.
Defined cpr_error_type_t, cpr_error_code_t, and cpr_error_t.
Defined the cpr_version_{major,minor,patch} symbols.
Implemented error callback hook invocation using the cpr_error(),
cpr_logic_error(), cpr_runtime_error(), and cpr_fatal_error() macros.
Renamed cpr_error_callback to cpr_error_hook.
Defined the cpr_error_sizeof symbol.
Ensured that ./configure checks for vsnprintf() availability.
Implemented cpr_abort() and cpr_abort_with_error().
Ensured that the C++11 'thread_local' keyword is used if supported.
Ensured that any README.html file is ignored by Git.
Added material on error-type distinctions to the README.
Added a standalone Makefile for building the example programs.
Corrected the main() declarations in the example programs.
Ensured that any *.dSYM build artifacts are ignored by Git.
Ensured that the `sizeof` program also reports sizeof(cpr_error_t).
Ensured that <cpr/error.h> includes <stdbool.h>.